diff --git a/client/cros/chameleon/chameleon_screen_test.py b/client/cros/chameleon/chameleon_screen_test.py
index 711378e..e6e5d96 100644
--- a/client/cros/chameleon/chameleon_screen_test.py
+++ b/client/cros/chameleon/chameleon_screen_test.py
@@ -114,7 +114,8 @@
                 if error is None:
                     return error
                 elif retry_count == 0:
-                    error_list.append(error)
+                    if error_list is not None:
+                        error_list.append(error)
                     return error
                 else:
                     logging.info('Retry screen comparison again...')