How prevent creating a base model in django-polymorphic

Question

I want to only be able to create new objects on sub-classes and not on base class.

class Product(PolymorphicModel):
    # fields

class ProductType1(Product):
    # fields

class ProductType2(Product):
    # fields

For example in above code creating new objects should only be possible on ProductType1 and ProductType2 and NOT on Product it self.