The Django 1.8 system check framework displays the warning fields.W340 when the null parameter is defined on a ManyToManyField, since it has no effect.

This null field affects on whether a related field in a model serializer is considered required or not. We can see it here.

From my point of view, this behavior is correct for a ForeignKey but not for a ManyToManyField, that should be considered not required by default.

Adding the var to_many to the condition on the aforementioned code might do the trick.