diff --git a/libs/core/langchain_core/indexing/api.py b/libs/core/langchain_core/indexing/api.py
index 75efe1f5659..b5528442fd3 100644
--- a/libs/core/langchain_core/indexing/api.py
+++ b/libs/core/langchain_core/indexing/api.py
@@ -410,7 +410,10 @@ def index(
             # here due to a check that's happening above, so we check again.
             for source_id in source_ids:
                 if source_id is None:
-                    msg = "Source ids cannot be None here."
+                    msg = (
+                        "source_id cannot be None at this point. "
+                        "Reached unreachable code."
+                    )
                     raise AssertionError(msg)
 
             _source_ids = cast(Sequence[str], source_ids)
@@ -670,7 +673,10 @@ async def aindex(
             # here due to a check that's happening above, so we check again.
             for source_id in source_ids:
                 if source_id is None:
-                    msg = "Source ids cannot be None here."
+                    msg = (
+                        "source_id cannot be None at this point. "
+                        "Reached unreachable code."
+                    )
                     raise AssertionError(msg)
 
             _source_ids = cast(Sequence[str], source_ids)