diff --git a/libs/core/langchain_core/tracers/log_stream.py b/libs/core/langchain_core/tracers/log_stream.py
index c6b8da943a..db56626451 100644
--- a/libs/core/langchain_core/tracers/log_stream.py
+++ b/libs/core/langchain_core/tracers/log_stream.py
@@ -572,6 +572,7 @@ async def _astream_log_implementation(
                     try:
                         final_output = final_output + chunk  # type: ignore
                     except TypeError:
+                        prev_final_output = None
                         final_output = chunk
                 patches: List[Dict[str, Any]] = []
                 if with_streamed_output_list: