Entity Framework does not cascade deletion of child objects

Question

I have parent and child objects. In db, the relation is set to `ON DELETE CASCADE '.

The first Fluent api EF coding for a relationship:

this.HasMany(t => t.Children)
    .WithOptional()
    .HasForeignKey(d => d.ParentId)
    .WillCascadeOnDelete();

Code for deleting an entity:

public virtual void Delete(Parent entity)
{
    DbEntityEntry dbEntityEntry = dataContext.GetEntry(entity);
    if (dbEntityEntry.State != EntityState.Deleted)
    {
        dbEntityEntry.State = EntityState.Deleted;
    }

    else
    {
        dbSet.Attach(entity);
        dbSet.Remove(entity);
    }
}

When I try to delete the parent, I get the following exception:

The operation failed: the relation cannot be changed because one or more properties of the foreign key are not NULL. When a change in relationship occurs, the corresponding property of the foreign key is set to zero. If the foreign key does not support null values, a new relationship must be defined, another nonzero value must be assigned to the foreign key property, or an object not associated with it must be deleted.

, EF ?

?

Delete:

if (entity.Children != null)
{
    dataContext.Set<Child>().RemoveRange(entity.Children);
}

.

? EF/SQL Server ?