diff --git a/tests/services/category_test.py b/tests/services/category_test.py
index eb010de..089e851 100644
--- a/tests/services/category_test.py
+++ b/tests/services/category_test.py
@@ -61,6 +61,7 @@ def test_get_category_by_id_with_none(app, mock_db, mock_category_service):
     with app.app_context():
         # Arrange
         category_id = None
+        mock_db.fetchone.return_value = None
 
         # Act
         result = mock_category_service.get_category_by_id(category_id)