diff --git a/returnn/tf/util/data.py b/returnn/tf/util/data.py
index f256fafda..3bfe8d6ec 100644
--- a/returnn/tf/util/data.py
+++ b/returnn/tf/util/data.py
@@ -283,6 +283,7 @@ def set_tag_on_size_tensor(self, x, batch=None, same_as_before=False):
       if x in self._dyn_size_same:
         pass  # ok, pass on
       elif same_as_before:
+        # TODO add runtime test
         self._dyn_size_same.add(x)
         # And now pass on.
       else: