diff --git a/llmfoundry/utils/exceptions.py b/llmfoundry/utils/exceptions.py
index 85f43f7fd1..76e8e719f5 100644
--- a/llmfoundry/utils/exceptions.py
+++ b/llmfoundry/utils/exceptions.py
@@ -361,9 +361,13 @@ def __init__(self, timeout: int) -> None:
 class DatasetMissingFileError(UserError):
     """Error thrown when a dataset cannot find a file."""
 
-    def __init__(self, file_name: str, supported_extensions: Optional[List[str]]) -> None:
+    def __init__(
+        self,
+        file_name: str,
+        supported_extensions: Optional[List[str]] = None,
+    ) -> None:
         message = "Could not find the file '{file_name}' with any of the supported extensions: "
-        if supported_extensions is not None:       
+        if supported_extensions is not None:
             message += ', '.join(supported_extensions) + '.'
         message += ' Please check your train / eval data and try again.'
         super().__init__(message, file_name=file_name)