diff --git a/src/mistralai/models/conversationusageinfo.py b/src/mistralai/models/conversationusageinfo.py
index 9ae6f4f..017b818 100644
--- a/src/mistralai/models/conversationusageinfo.py
+++ b/src/mistralai/models/conversationusageinfo.py
@@ -28,35 +28,35 @@ class ConversationUsageInfo(BaseModel):
 
     @model_serializer(mode="wrap")
     def serialize_model(self, handler):
-        optional_fields = [
+        optional_fields = (
             "prompt_tokens",
             "completion_tokens",
             "total_tokens",
             "connector_tokens",
             "connectors",
-        ]
-        nullable_fields = ["connector_tokens", "connectors"]
+        )
+        nullable_fields = {"connector_tokens", "connectors"}
         null_default_fields = []
 
         serialized = handler(self)
 
         m = {}
 
-        for n, f in type(self).model_fields.items():
+        fields_set = self.__pydantic_fields_set__  # pylint: disable=no-member
+        model_fields = type(self).model_fields
+
+        for n, f in model_fields.items():
             k = f.alias or n
             val = serialized.get(k)
             serialized.pop(k, None)
 
             optional_nullable = k in optional_fields and k in nullable_fields
-            is_set = (
-                self.__pydantic_fields_set__.intersection({n})
-                or k in null_default_fields
-            )  # pylint: disable=no-member
+            is_set = n in fields_set or k in null_default_fields
 
             if val is not None and val != UNSET_SENTINEL:
                 m[k] = val
             elif val != UNSET_SENTINEL and (
-                not k in optional_fields or (optional_nullable and is_set)
+                k not in optional_fields or (optional_nullable and is_set)
             ):
                 m[k] = val