[Bug] URDF Importer crashes with 'Used null prim' on URDFs with empty base_footprint root link

[xteo]

Description

The URDF importer (URDFParseAndImportFile) crashes with RuntimeError: Used null prim when importing URDFs that have an empty root link (e.g., base_footprint) — a very common pattern in ROS robot descriptions.

Steps to Reproduce

1. Use Isaac Sim 6.0.0-rc.13 (built from source on ARM64/GB300)
2. Import any URDF with an empty root link pattern:

<link name="base_footprint"/>
<joint name="base_joint" type="fixed">
  <parent link="base_footprint"/>
  <child link="base_link"/>
</joint>

3. Both merge_fixed_joints=True and merge_fixed_joints=False crash

Error Output

[Warning] [isaacsim.asset.importer.urdf] Link base_footprint has no colliders, and no inertia was imported; assigning a small isotropic inertia matrix
[Warning] [omni.usd] Coding Error: in _IsValidPathForCreatingPrim at line 3720 -- Path must be an absolute path: <>
[Error] Failed to execute a command: URDFParseAndImportFile.
RuntimeError: Used null prim

Expected Behavior

The importer should handle empty root links gracefully (skip them, merge into the first child with geometry, or treat as the articulation root without geometry).

Environment

• Isaac Sim 6.0.0-rc.13 (built from develop branch)
• Platform: ARM64 (NVIDIA DGX Station GB300)
• CUDA 13.1 / Driver 590.48.01
• Ubuntu 24.04 (Noble Numbat)

Workaround

Convert STL meshes to USD manually using omni.kit.asset_converter, then assemble the robot in USD with correct transforms from the URDF. Bypasses the URDF importer entirely.

Affected Robot

MiR250 (Mobile Industrial Robots) — uses standard ROS base_footprint → base_link pattern. The DFKI-NI/mir_robot ROS package URDF fails to import.

[vick-yu]

Thanks for the clear report and repro steps. Could you check whether this still reproduces in Isaac Sim 6.0-rc.22 (current repo version)?

[VIPUL577]

Yes, in the current repo the same issue is occurring. I even tried removing the empty link, and my urdf works in online URDF viewer.

[vick-yu]

Hi @VIPUL577,

Thanks for confirming this is still happening. To help us investigate further, could you share:

1. Your URDF file (or a minimal version that reproduces the crash) — you can attach it here or link to the repository
2. Your Isaac Sim version and platform — are you using the pip install, binary, or built from source? x86_64 or ARM64?
3. The full error output from your console/terminal
4. A quick test — can you try importing the bundled test URDF to confirm your setup is working for simpler cases?

      from isaacsim.asset.importer.urdf import URDFImporter, URDFImporterConfig                                                                                                                                                                                                                                              
                                                                                                                                                                                                                                                                                                                             
      config = URDFImporterConfig()                                                                                                                                                                                                                                                                                          
      config.urdf_path = "<ISAAC_SIM_PATH>/exts/isaacsim.asset.importer.urdf/data/urdf/tests/test_basic.urdf"                                                                                                                                                                                                                
                                                                                                                                                                                                                                                                                                                             
      importer = URDFImporter(config=config)                                                                                                                                                                                                                                                                                 
      result = importer.import_urdf()                                                                                                                                                                                                                                                                                        
      print(f"Import result: {result}")                                                                                                                                                                                                                                                                                      

This will help us determine whether the issue is specific to certain URDF structures (e.g., external mesh references, complex link hierarchies) or a broader regression in the importer.

Note: test_basic.urdf in our test suite also uses an empty root link pattern () — if that imports successfully but your URDF doesn't, the difference will help us pinpoint the exact trigger.

[VIPUL577]

this is the URDF location, just made a repo for URDF as the original Repo is private

https://github.com/VIPUL577/URDF_ISAACSIM_ISSUE

isaac sim version 5.1.0
Ubuntu 22.04 LTS
RTX4050

output from Console when using IMPORT IN GUI

2026-04-01T16:13:53Z [23,859ms] [Error] [omni.kit.commands.command] Failed to execute a command: URDFImportRobot.
File "/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/exts/isaacsim.asset.importer.urdf/isaacsim/asset/importer/urdf/impl/extension.py", line 461, in overwrite_callback
self._load_robot(dest_path, path)
File "/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/exts/isaacsim.asset.importer.urdf/isaacsim/asset/importer/urdf/impl/extension.py", line 588, in _load_robot
omni.kit.commands.execute(
File "/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/extscache/omni.kit.commands-1.4.10+69cbf6ad.lx64.r.cp311/omni/kit/commands/command.py", line 463, in execute
result = omni.kit.undo.execute(command, name, kwargs)
[...skipped...]
File "/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/extscache/omni.kit.commands-1.4.10+69cbf6ad.lx64.r.cp311/omni/kit/undo/undo.py", line 78, in execute
result = _execute(command, name, level, history_key)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/extscache/omni.kit.commands-1.4.10+69cbf6ad.lx64.r.cp311/omni/kit/undo/undo.py", line 459, in _execute
raise error
File "/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/extscache/omni.kit.commands-1.4.10+69cbf6ad.lx64.r.cp311/omni/kit/undo/undo.py", line 420, in _execute
result = command.do()
^^^^^^^^^^^^
File "/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/exts/isaacsim.asset.importer.urdf/isaacsim/asset/importer/urdf/impl/commands.py", line 145, in do
return self._urdf_interface.import_robot(
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
<class 'RuntimeError'> Used null prim
2026-04-01T16:13:53Z [23,860ms] [Error] [omni.ui.python] RuntimeError: Accessed invalid null prim
At:
/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/exts/isaacsim.asset.importer.urdf/isaacsim/asset/importer/urdf/impl/extension.py(596): _load_robot
/home/vipulagarwal/isaacsim/_build/linux-x86_64/release/exts/isaacsim.asset.importer.urdf/isaacsim/asset/importer/urdf/impl/extension.py(461): overwrite_callback

Please help me, stuck here for a week

[vick-yu]

Thanks for the details and the repro repo. One important clarification: you’re on Isaac Sim 5.1.0. The current issue was originally filed for 6.0.0-rc.13; 6.0 has since switched to a different URDF pipeline, so 5.1 and 6.0 are not the same code path.

Could you test with the latest v6.0.0-dev2 build?

[VIPUL577]

The thing is my issue got solved, there was a small error in stl files , had to remove empty links and a small naming issue. 😅😅

Thanks for your time though, would love to discuss projects with you

If you could share your contact.

Thanks !

image

[vick-yu]

Closing — fixed in v6.0.0-rc.22+

We reproduced the reported scenario (URDF with an empty base_footprint root link, which is a standard ROS pattern) and can confirm:

• v6.0.0-rc.13 — crashes with RuntimeError: Used null prim as reported
• v6.0.0-rc.22 — import completes successfully, no crash

The empty root link is now handled correctly by the URDF importer. The fix landed somewhere between rc.13 and rc.22.

Test URDF used:

  <?xml version="1.0" encoding="UTF-8"?>                                                                                                                                                                                                                                                                                                                                                                                                
  <robot name="test_base_footprint">                                                                                                                                                                                                                                                                                                                                                                                                    
    <link name="base_footprint"/>                                                                                                                                                                                                                                                                                                                                                                                                       
    <joint name="base_joint" type="fixed">                                                                                                                                                                                                                                                                                                                                                                                              
      <parent link="base_footprint"/>                                                                                                                                                                                                                                                                                                                                                                                                   
      <child link="base_link"/>                                                                                                                                                                                                                                                                                                                                                                                                         
    </joint>                                                                                                                                                                                                                                                                                                                                                                                                                            
    <link name="base_link">                                                                                                                                                                                                                                                                                                                                                                                                             
      <visual><geometry><box size="0.5 0.3 0.1"/></geometry></visual>                                                                                                                                                                                                                                                                                                                                                                   
      <collision><geometry><box size="0.5 0.3 0.1"/></geometry></collision>                                                                                                                                                                                                                                                                                                                                                             
      <inertial>                                                                                                                                                                                                                                                                                                                                                                                                                        
        <mass value="10"/>                                                                                                                                                                                                                                                                                                                                                                                                              
        <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>                                                                                                                                                                                                                                                                                                                                                          
      </inertial>                                                                                                                                                                                                                                                                                                                                                                                                                       
    </link>                                                                                                                                                                                                                                                                                                                                                                                                                             
    <joint name="wheel_joint" type="continuous">                                                                                                                                                                                                                                                                                                                                                                                        
      <parent link="base_link"/>                                                                                                                                                                                                                                                                                                                                                                                                        
      <child link="wheel_link"/>                                                                                                                                                                                                                                                                                                                                                                                                        
      <axis xyz="0 1 0"/>                                                                                                                                                                                                                                                                                                                                                                                                               
      <origin xyz="0.2 0 -0.05"/>                                                                                                                                                                                                                                                                                                                                                                                                       
    </joint>                                                                                                                                                                                                                                                                                                                                                                                                                            
    <link name="wheel_link">                                                                                                                                                                                                                                                                                                                                                                                                            
      <visual><geometry><cylinder length="0.04" radius="0.05"/></geometry></visual>                                                                                                                                                                                                                                                                                                                                                     
      <collision><geometry><cylinder length="0.04" radius="0.05"/></geometry></collision>                                                                                                                                                                                                                                                                                                                                               
      <inertial>                                                                                                                                                                                                                                                                                                                                                                                                                        
        <mass value="1"/>                                                                                                                                                                                                                                                                                                                                                                                                               
        <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01"/>                                                                                                                                                                                                                                                                                                                                                       
      </inertial>                                                                                                                                                                                                                                                                                                                                                                                                                       
    </link>                                                                                                                                                                                                                                                                                                                                                                                                                             
  </robot>                                                                                                                                                                                                                                                                                                                                                                                                                              

@reporter — if you're still on rc.13, upgrading to the latest 6.0.0 RC should resolve this. If you can reproduce on a newer RC with your MiR250 URDF specifically, please reopen.