3.15 is raising required error on model nullable fields

[Enorio]

I have a model with nullable fields. Upgrading from 3.14 to 3.15 I'm getting this error:
AssertionError: You cannot call .save() on a serializer with invalid data.

When I print the serializer.errors after serializer.is_valid(), it says those fields are required. Is it possible that this is a bug, or do I have to manually override the field in the serializer to have the required=False kwarg?

[sevdog]

Could you provide a code sample of your model, serializer and data for checking?