diff --git a/geojson_pydantic/base.py b/geojson_pydantic/base.py
index 299cfb8..c7f7936 100644
--- a/geojson_pydantic/base.py
+++ b/geojson_pydantic/base.py
@@ -53,7 +53,7 @@ def validate_bbox(cls, bbox: Optional[BBox]) -> Optional[BBox]:
         return bbox
 
     @model_serializer(when_used="always", mode="wrap")
-    def clean_model(self, serializer: Any, info: SerializationInfo):
+    def clean_model(self, serializer: Any, info: SerializationInfo) -> Any:
         """Custom Model serializer to match the GeoJSON specification.
 
         Used to remove fields which are optional but cannot be null values.