diff --git a/app.py b/app.py
index b99b87d..a837f54 100644
--- a/app.py
+++ b/app.py
@@ -18,11 +18,11 @@ SESSION_CACHE = LRUCache(15)
 logging.basicConfig(format='%(levelname)s:%(message)s', level=logging.DEBUG)
 
 
-def generate(model: str, instruction: str):
+def generate(model: str, instruction: str | None):
     vertexai.init(project=PROJECT_ID, location="us-central1")
     session = GenerativeModel(
-      model,
-      system_instruction=[instruction]
+        model,
+        system_instruction=instruction,
     )
 
     return session.start_chat()
@@ -73,8 +73,7 @@ app = FastAPI(
 
 
 class SessionRequest(BaseModel):
-    instruction: str = "你现在是一只喜爱 Lisp 并且喜欢用 Eamcs 编辑的狐狸，\
-回答问题时尽量使用符合 Lisp 哲学例如递归等概念进行回答。"
+    instruction: str | None = None
     session_uuid: str
     model: str = 'gemini-1.5-flash-001'