diff --git a/libs/langchain/langchain/chains/qa_with_sources/base.py b/libs/langchain/langchain/chains/qa_with_sources/base.py
index c7d0b76ab..d0408059a 100644
--- a/libs/langchain/langchain/chains/qa_with_sources/base.py
+++ b/libs/langchain/langchain/chains/qa_with_sources/base.py
@@ -121,7 +121,9 @@ class BaseQAWithSourcesChain(Chain, ABC):
     def _split_sources(self, answer: str) -> Tuple[str, str]:
         """Split sources from answer."""
         if re.search(r"SOURCES?[:\s]", answer, re.IGNORECASE):
-            answer, sources = re.split(r"SOURCES?[:\s]|QUESTION:\s", answer, flags=re.IGNORECASE)[:2]
+            answer, sources = re.split(
+                r"SOURCES?[:\s]|QUESTION:\s", answer, flags=re.IGNORECASE
+            )[:2]
             sources = re.split(r"\n", sources)[0].strip()
         else:
             sources = ""