diff --git a/cms/grading/tasktypes/Batch.py b/cms/grading/tasktypes/Batch.py
index 9de82c823d..66ebd21001 100644
--- a/cms/grading/tasktypes/Batch.py
+++ b/cms/grading/tasktypes/Batch.py
@@ -195,7 +195,7 @@ def _executable_filename(codenames: Iterable[str], language: Language) -> str:
 
         """
         name = "_".join(sorted(codename.replace(".%l", "")
-                               for codename in codenames))
+                               for codename in codenames if codename.endswith(".%l")))
         return name + language.executable_extension
 
     def _do_compile(self, job: CompilationJob, file_cacher: FileCacher):