diff --git a/python/sglang/lang/ir.py b/python/sglang/lang/ir.py
index 2e81d4bcd..75f4d0bb6 100644
--- a/python/sglang/lang/ir.py
+++ b/python/sglang/lang/ir.py
@@ -150,7 +150,7 @@ class SglFunction:
         self,
         *args,
         max_new_tokens: int = 128,
-        stop: Union[str, List[str]] = None,
+        stop: Optional[Union[str, List[str]]] = None,
         stop_token_ids: Optional[List[int]] = None,
         temperature: float = 1.0,
         top_p: float = 1.0,
@@ -199,7 +199,7 @@ class SglFunction:
         batch_kwargs,
         *,
         max_new_tokens: int = 128,
-        stop: Union[str, List[str]] = None,
+        stop: Optional[Union[str, List[str]]] = None,
         stop_token_ids: Optional[List[int]] = None,
         temperature: float = 1.0,
         top_p: float = 1.0,
diff --git a/python/sglang/srt/sampling/sampling_params.py b/python/sglang/srt/sampling/sampling_params.py
index 2c251bac4..700fefa3d 100644
--- a/python/sglang/srt/sampling/sampling_params.py
+++ b/python/sglang/srt/sampling/sampling_params.py
@@ -41,8 +41,6 @@ class SamplingParams:
         n: int = 1,
         json_schema: Optional[str] = None,
     ) -> None:
-        if stop_token_ids is None:
-            stop_token_ids = []
         self.temperature = temperature
         self.top_p = top_p
         self.top_k = top_k
@@ -51,6 +49,8 @@ class SamplingParams:
         self.presence_penalty = presence_penalty
         self.repetition_penalty = repetition_penalty
         self.stop_strs = stop
+        if stop_token_ids is None:
+            stop_token_ids = []
         self.stop_token_ids = {*stop_token_ids}
         self.max_new_tokens = max_new_tokens
         self.min_new_tokens = min_new_tokens