diff --git a/api/core/workflow/nodes/document_extractor/node.py b/api/core/workflow/nodes/document_extractor/node.py
index 3580f17cbb..e2faae2f4d 100644
--- a/api/core/workflow/nodes/document_extractor/node.py
+++ b/api/core/workflow/nodes/document_extractor/node.py
@@ -468,8 +468,7 @@ def _extract_text_from_msg(file_content: bytes) -> str:
     except Exception as e:
         raise TextExtractionError(f"Failed to extract text from MSG: {str(e)}") from e
 
-
-def _extract_text_from_vtt(vtt_bytes: bytes):
+def _extract_text_from_vtt(vtt_bytes: bytes) -> str:
     text = _extract_text_from_plain_text(vtt_bytes)
 
     # remove bom