The Django 1.8 system check framework displays the warningfields.W340 when thenull parameter is defined on aManyToManyField, since it has no effect.

Thisnull field affects on whether a related field in a model serializer is considered required or not. We can see ithere.

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

Adding the varto_many to the condition on the aforementioned code might do the trick.