diff --git a/metagpt/schema.py b/metagpt/schema.py
index 749e0fd56..2e4a6c62f 100644
--- a/metagpt/schema.py
+++ b/metagpt/schema.py
@@ -9,7 +9,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass, field
-from typing import Type, TypedDict, Set
+from typing import Type, TypedDict, Set, Optional
 
 from pydantic import BaseModel
 
@@ -30,7 +30,7 @@ class Message:
     cause_by: Type["Action"] = field(default="")
     sent_from: str = field(default="")
     send_to: str = field(default="")
-    tags: Set = field(default_factory=set())
+    tags: Optional[Set] = field(default=None)
 
     def __str__(self):
         # prefix = '-'.join([self.role, str(self.cause_by)])