diff --git a/test/.excludes-mmtk/TestObjSpace.rb b/test/.excludes-mmtk/TestObjSpace.rb
index 94eb2c436d4435..703efd79bb8c69 100644
--- a/test/.excludes-mmtk/TestObjSpace.rb
+++ b/test/.excludes-mmtk/TestObjSpace.rb
@@ -1,4 +1,5 @@
 exclude(:test_dump_all_full, "testing behaviour specific to default GC")
 exclude(:test_dump_flag_age, "testing behaviour specific to default GC")
 exclude(:test_dump_flags, "testing behaviour specific to default GC")
+exclude(:test_dump_includes_slot_size, "can be removed when BASE_SLOT_SIZE is 32 bytes")
 exclude(:test_dump_objects_dumps_page_slot_sizes, "testing behaviour specific to default GC")
diff --git a/test/objspace/test_objspace.rb b/test/objspace/test_objspace.rb
index 8b369c38945f4c..d631f97d1bcad8 100644
--- a/test/objspace/test_objspace.rb
+++ b/test/objspace/test_objspace.rb
@@ -703,8 +703,9 @@ def test_dump_escapes_method_name
   end
 
   def test_dump_includes_slot_size
-    str = "TEST"
-    dump = ObjectSpace.dump(str)
+    klass = Class.new
+    obj = klass.new
+    dump = ObjectSpace.dump(obj)
 
     assert_includes dump, "\"slot_size\":#{GC::INTERNAL_CONSTANTS[:BASE_SLOT_SIZE]}"
   end