diff --git a/jupyter_ydoc/ynotebook.py b/jupyter_ydoc/ynotebook.py
index 8099497..c117b44 100644
--- a/jupyter_ydoc/ynotebook.py
+++ b/jupyter_ydoc/ynotebook.py
@@ -304,7 +304,7 @@ def set(self, value: Dict) -> None:
                 self._ymeta["nbformat_minor"] = nbformat_minor
 
             old_y_metadata = self._ymeta.get("metadata")
-            old_metadata = old_y_metadata.to_py() if old_y_metadata else {}
+            old_metadata = old_y_metadata.to_py() if old_y_metadata else None
             metadata = nb.get("metadata", {})
 
             if metadata != old_metadata:
diff --git a/tests/test_ynotebook.py b/tests/test_ynotebook.py
index 282133e..5ff293c 100644
--- a/tests/test_ynotebook.py
+++ b/tests/test_ynotebook.py
@@ -44,6 +44,15 @@ def record_changes(topic, event):
     assert changes == []
 
 
+def test_set_populates_metadata():
+    nb = YNotebook()
+    nb.set({"cells": []})
+    assert nb.get()["metadata"] == {
+        "kernelspec": {"display_name": "", "name": ""},
+        "language_info": {"name": ""},
+    }
+
+
 def test_set_preserves_cells_with_insert_and_remove():
     nb = YNotebook()
     nb.set(