diff --git a/src/jsonfield/forms.py b/src/jsonfield/forms.py
index 02cd563..230033f 100644
--- a/src/jsonfield/forms.py
+++ b/src/jsonfield/forms.py
@@ -59,6 +59,6 @@ def bound_data(self, data, initial):
             return InvalidJSONInput(data)
 
     def prepare_value(self, value):
-        if isinstance(value, InvalidJSONInput):
+        if isinstance(value, InvalidJSONInput) or value is None:
             return value
         return json.dumps(value, **self.dump_kwargs)