MonitorField fails when a used on a required ForeignKey field

[owais]

class MyModel(models.Model):
    my_rel = models.ForeignKey('OtherModel')
    my_rel_changed = MonitorField(monitor='my_rel')

m = MyModel()
>> DoesNotExist: MyModel has no my_rel.

It'll work if I change my_rel to

    my_rel = models.ForeignKey('OtherModel', null=True)

The exception is raised here https://github.com/carljm/django-model-utils/blob/master/model_utils/fields.py#L105

def get_monitored_value(self, instance):
    return getattr(instance, self.monitor)