diff --git a/tsflex/features/feature_collection.py b/tsflex/features/feature_collection.py
index 7798a91..407cf86 100644
--- a/tsflex/features/feature_collection.py
+++ b/tsflex/features/feature_collection.py
@@ -223,7 +223,7 @@ def _add_feature(self, feature: FeatureDescriptor) -> None:
             assert not any(
                 output_name in added_output_names
                 for output_name in feature.function.output_names
-            )
+            ), f"Feature with output name(s) {feature.function.output_names} duplicated"
             self._feature_desc_dict[series_win_stride_key].append(feature)
         else:
             self._feature_desc_dict[series_win_stride_key] = [feature]