Django delete superclass update or delete on table violates foreign key constraint

I have two django models, one subclassing another,

class A(models.Model) : 
    .
    .
class B(A) : 
    .
    .

the problem is when I'm deleting class A I'm getting

update or delete on table "a" violates foreign key constraint "b_a_ptr_id_53f8796d_fk_a" on table "b"

Shouldn't the delete automaticly take out the row on table B pointing to the deleted element? Can anyone guide me through this please.