diff --git a/pyannote/core/timeline.py b/pyannote/core/timeline.py
index 1af9151..4a2559a 100755
--- a/pyannote/core/timeline.py
+++ b/pyannote/core/timeline.py
@@ -1034,7 +1034,7 @@ def segmentation(self) -> 'Timeline':
         return Timeline(segments=segments, uri=self.uri)
 
     def to_annotation(self,
-                      generator: Union[str, Iterable[Label], None, None] = 'string',
+                      generator: Union[str, Iterable[Label]] = 'string',
                       modality: Optional[str] = None) \
             -> 'Annotation':
         """Turn timeline into an annotation