diff --git a/gptc/model.py b/gptc/model.py
index bd64761..53bcf01 100644
--- a/gptc/model.py
+++ b/gptc/model.py
@@ -3,7 +3,7 @@
 import gptc.tokenizer
 from gptc.exceptions import InvalidModelError
 import gptc.weighting
-from typing import Iterable, Mapping, List, Dict, Union, cast
+from typing import Iterable, Mapping, List, Dict, Union, cast, BinaryIO
 import json
 
 
@@ -76,7 +76,7 @@ class Model:
             for index, category in enumerate(self.names)
         }
 
-    def serialize(self, file):
+    def serialize(self, file: BinaryIO):
         file.write(b"GPTC model v4\n")
         file.write(
             json.dumps(