diff --git a/simplemind/models.py b/simplemind/models.py
index 47c57e8..9019de1 100644
--- a/simplemind/models.py
+++ b/simplemind/models.py
@@ -79,9 +79,8 @@ class Conversation(SMBaseModel):
     def __str__(self):
         return f"<Conversation id={self.id!r}>"
 
-    def prepend_system_message(self, role: str, text: str, meta: Dict[str, Any] = {}):
-        self.messages = [Message(role=role, text=text, meta=meta)] + self.messages
-
+    def prepend_system_message(self, role: str, text: str, meta: Optional[Dict[str, Any]] = None):
+        self.messages = [Message(role=role, text=text, meta=meta or {})] + self.messages
     def add_message(self, role: str, text: str, meta: Dict[str, Any] = {}):
         """Add a new message to the conversation."""
         self.messages.append(Message(role=role, text=text, meta=meta))