diff --git a/src/mistralai/models/audiotranscriptionrequeststream.py b/src/mistralai/models/audiotranscriptionrequeststream.py
index 0437450..63195dd 100644
--- a/src/mistralai/models/audiotranscriptionrequeststream.py
+++ b/src/mistralai/models/audiotranscriptionrequeststream.py
@@ -60,7 +60,7 @@ class AudioTranscriptionRequestStream(BaseModel):
 
     @model_serializer(mode="wrap")
     def serialize_model(self, handler):
-        optional_fields = [
+        optional_fields = {
             "file",
             "file_url",
             "file_id",
@@ -68,9 +68,9 @@ def serialize_model(self, handler):
             "temperature",
             "stream",
             "timestamp_granularities",
-        ]
-        nullable_fields = ["file_url", "file_id", "language", "temperature"]
-        null_default_fields = []
+        }
+        nullable_fields = {"file_url", "file_id", "language", "temperature"}
+        null_default_fields = set()
 
         serialized = handler(self)
 
@@ -79,18 +79,14 @@ def serialize_model(self, handler):
         for n, f in type(self).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 self.__pydantic_fields_set__ or k in null_default_fields  # pylint: disable=no-member
 
             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