diff --git a/torchaudio/prototype/ctc_decoder/ctc_decoder.py b/torchaudio/prototype/ctc_decoder/ctc_decoder.py
index 3c0bf1883d..567dcbe659 100644
--- a/torchaudio/prototype/ctc_decoder/ctc_decoder.py
+++ b/torchaudio/prototype/ctc_decoder/ctc_decoder.py
@@ -181,7 +181,7 @@ def idxs_to_tokens(self, idxs: torch.LongTensor) -> List:
 def lexicon_decoder(
     lexicon: str,
     tokens: Union[str, List[str]],
-    lm: str = None,
+    lm: Optional[str] = None,
     nbest: int = 1,
     beam_size: int = 50,
     beam_size_token: Optional[int] = None,