diff --git a/src/google/adk/integrations/firestore/firestore_session_service.py b/src/google/adk/integrations/firestore/firestore_session_service.py
index 7371b51a96a..ece0aae20dd 100644
--- a/src/google/adk/integrations/firestore/firestore_session_service.py
+++ b/src/google/adk/integrations/firestore/firestore_session_service.py
@@ -368,7 +368,11 @@ async def list_sessions(
       self, *, app_name: str, user_id: Optional[str] = None
   ) -> ListSessionsResponse:
     """Lists sessions from Firestore."""
-    if user_id:
+    if user_id == "":
+      # Firestore rejects a document path with an empty segment, so no session
+      # can be stored for this user, and querying for one would fail.
+      return ListSessionsResponse()
+    if user_id is not None:
       query = self._get_sessions_ref(app_name, user_id).where(
           filter=FieldFilter("appName", "==", app_name)
       )
@@ -392,7 +396,7 @@ def _iter_sessions_data() -> Iterator[dict[str, Any]]:
     app_state = app_doc.to_dict() if app_doc.exists else {}
 
     user_states_map = {}
-    if user_id:
+    if user_id is not None:
       user_ref = (
           self.client.collection(self.user_state_collection)
           .document(app_name)
diff --git a/tests/unittests/integrations/firestore/test_firestore_session_service.py b/tests/unittests/integrations/firestore/test_firestore_session_service.py
index ec90d3c3809..f3055aa8c02 100644
--- a/tests/unittests/integrations/firestore/test_firestore_session_service.py
+++ b/tests/unittests/integrations/firestore/test_firestore_session_service.py
@@ -961,6 +961,22 @@ async def mock_get_all(refs):
   assert _where_filter(where) == ("appName", "==", app_name)
 
 
+@pytest.mark.asyncio
+async def test_list_sessions_with_empty_user_id_returns_no_sessions(
+    mock_firestore_client,
+):
+  """user_id="" must not fall through to the all-users query."""
+  service = FirestoreSessionService(client=mock_firestore_client)
+  mock_firestore_client.collection_group.return_value.where.return_value.get = (
+      mock.AsyncMock(return_value=[])
+  )
+
+  response = await service.list_sessions(app_name="test_app", user_id="")
+
+  assert response.sessions == []
+  mock_firestore_client.collection_group.assert_not_called()
+
+
 @pytest.mark.asyncio
 async def test_get_user_state_returns_stored_user_state(mock_firestore_client):
   """get_user_state returns the user_states document for the app and user."""