diff --git a/libs/core/langchain_core/messages/utils.py b/libs/core/langchain_core/messages/utils.py
index 32207673747..b22c927ad4d 100644
--- a/libs/core/langchain_core/messages/utils.py
+++ b/libs/core/langchain_core/messages/utils.py
@@ -503,8 +503,11 @@ def merge_message_runs(
         else:
             last_chunk = _msg_to_chunk(last)
             curr_chunk = _msg_to_chunk(curr)
-            if isinstance(last_chunk.content, str) and isinstance(
-                curr_chunk.content, str
+            if (
+                isinstance(last_chunk.content, str)
+                and isinstance(curr_chunk.content, str)
+                and last_chunk.content
+                and curr_chunk.content
             ):
                 last_chunk.content += "\n"
             merged.append(_chunk_to_msg(last_chunk + curr_chunk))