Django 1.7: how to make ManyToManyField required?

I have a Django Model with ManyToManyField in it. I need to require user to select at least one M2M value in this field.

I tried to set blank=False to M2M field but it didn't help.

class Skill(models.Model):
    name = models.CharField(max_length=200)

class PersonSkills(models.Model):
    person = models.ForeignKey('Person')
    skill = models.ForeignKey('Skill')

class Person(models.Model):
    name = models.CharField(max_length=200)
    skills = models.ManyToManyField('Skill', through='PersonSkills')

p = Person(name='Bob')
p.save()
# success, but I expect that this should throw ValidationError, because I didn't select at least one Skill for this person

I can solve this situation with custom Form definition or with override save() method for Person model.

Is it possible to prevent create Person without at least one Skill selected, with set ManyToManyField options? Or I need to create custom logic to handle this situation? Thanks.

I use Django 1.7 and Python 3.4