diff --git a/metagpt/schema.py b/metagpt/schema.py
index 94e64d7fa..de49c8886 100644
--- a/metagpt/schema.py
+++ b/metagpt/schema.py
@@ -669,7 +669,7 @@ class Plan(BaseModel):
         """
         return [task for task in self.tasks if task.is_finished]
 
-    def append_task(self, task_id: str, dependent_task_ids: list[str], instruction: str, assignee: str, task_type: str):
+    def append_task(self, task_id: str, dependent_task_ids: list[str], instruction: str, assignee: str, task_type: str = ""):
         """Append a new task with task_id (number) to the end of existing task sequences. If dependent_task_ids is not empty, the task will depend on the tasks with the ids in the list."""
         new_task = Task(
             task_id=task_id,