diff --git a/tools/test_build_system/conftest.py b/tools/test_build_system/conftest.py
index 0fb2ec88673..ad789dcb82d 100644
--- a/tools/test_build_system/conftest.py
+++ b/tools/test_build_system/conftest.py
@@ -67,10 +67,13 @@ def _create_idf_copy_via_worktree(path_from: Path, path_to: Path) -> str:
         src_submodule = path_from / submodule_rel_path
         dst_submodule = path_to / submodule_rel_path
 
-        # Only copy if source submodule exists and has content
-        if src_submodule.exists() and any(src_submodule.iterdir()):
+        # Only copy if the source submodule is a populated directory. A gitlink
+        # file or empty dir means the source checkout did not materialize it.
+        if src_submodule.is_dir() and any(src_submodule.iterdir()):
             logging.debug(f'copying submodule {submodule_rel_path}')
-            # Worktree submodule paths may be gitlink files; rmtree() does not remove those.
+            # Worktree submodule paths are often gitlink files; rmtree() cannot
+            # remove those (even with ignore_errors=True), and copytree() then
+            # leaves a file where CMake expects a directory (e.g. mbedtls/include).
             if dst_submodule.is_file() or dst_submodule.is_symlink():
                 dst_submodule.unlink()
             elif dst_submodule.exists():