diff --git a/metagpt/memory/brain_memory.py b/metagpt/memory/brain_memory.py
index 92a71f69a..2195da566 100644
--- a/metagpt/memory/brain_memory.py
+++ b/metagpt/memory/brain_memory.py
@@ -34,7 +34,7 @@ class BrainMemory(pydantic.BaseModel):
     historical_summary: str = ""
     last_history_id: str = ""
     is_dirty: bool = False
-    last_talk: str = ""
+    last_talk: str = None
 
     def add_talk(self, msg: Message):
         msg.add_tag(MessageType.Talk.value)
@@ -127,5 +127,5 @@ class BrainMemory(pydantic.BaseModel):
 
     def pop_last_talk(self):
         v = self.last_talk
-        self.last_talk = ""
+        self.last_talk = None
         return v
diff --git a/metagpt/roles/assistant.py b/metagpt/roles/assistant.py
index 4b2bfdab5..87127cbab 100644
--- a/metagpt/roles/assistant.py
+++ b/metagpt/roles/assistant.py
@@ -120,7 +120,7 @@ class Assistant(Role):
 
     async def refine_memory(self) -> str:
         history_text = self.memory.history_text
-        last_talk = self.memory.last_talk
+        last_talk = self.memory.pop_last_talk()
         if last_talk is None:  # No user feedback, unsure if past conversation is finished.
             return None
         if history_text == "":