diff --git a/src/databricks/labs/ucx/source_code/notebooks/sources.py b/src/databricks/labs/ucx/source_code/notebooks/sources.py
index 5889166f89..1a71b21a37 100644
--- a/src/databricks/labs/ucx/source_code/notebooks/sources.py
+++ b/src/databricks/labs/ucx/source_code/notebooks/sources.py
@@ -198,10 +198,12 @@ def _load_tree_from_python_cell(self, python_cell: PythonCell, register_trees: b
         maybe_tree = Tree.maybe_normalized_parse(python_cell.original_code)
         if maybe_tree.failure:
             yield maybe_tree.failure
-        assert maybe_tree.tree is not None
         tree = maybe_tree.tree
+        # a cell with only comments will not produce a tree
         if register_trees:
-            self._python_trees[python_cell] = tree
+            self._python_trees[python_cell] = tree or Tree.new_module()
+        if not tree:
+            return
         yield from self._load_children_from_tree(tree)
 
     def _load_children_from_tree(self, tree: Tree) -> Iterable[Advice]: