diff --git a/src/datasets/download/mock_download_manager.py b/src/datasets/download/mock_download_manager.py
index 4367a147a8b..a4dd0e2ad22 100644
--- a/src/datasets/download/mock_download_manager.py
+++ b/src/datasets/download/mock_download_manager.py
@@ -161,8 +161,9 @@ def create_dummy_data_dict(self, path_to_dummy_data, data_url):
             dummy_data_dict[key] = value
 
         # make sure that values are unique
-        first_value = next(iter(dummy_data_dict.values()))
-        if isinstance(first_value, str) and len(set(dummy_data_dict.values())) < len(dummy_data_dict.values()):
+        if all([isinstance(i, str) for i in dummy_data_dict.values()]) and len(set(dummy_data_dict.values())) < len(
+            dummy_data_dict.values()
+        ):
             # append key to value to make its name unique
             dummy_data_dict = {key: value + key for key, value in dummy_data_dict.items()}