diff --git a/cassis/json.py b/cassis/json.py
index 370188f..506df1b 100644
--- a/cassis/json.py
+++ b/cassis/json.py
@@ -268,7 +268,7 @@ def serialize(
             json_fs = self._serialize_feature_structure(fs)
             feature_structures.append(json_fs)
 
-        if not isinstance(sink, TextIOBase):
+        if sink and not isinstance(sink, TextIOBase):
             sink = TextIOWrapper(sink, encoding="utf-8", write_through=True)
 
         if sink: