Possible Bug - Cycle, Blend using a generator of None never ends

[giovanisleite]

When I use a generator of None for a FloatField and run python manage.py test hangs at System check identified no issues (0 silenced). Using django shell, the command never ends.

# models.py
class Product(models.Model):
    price = models.FloatField(null=True)
    (...)

# test.py
mixer.cycle(3).blend(Product, price=(None for _ in range(3)))

Am I doing something wrong?