BooleanField default value not set when creating a model instance

When creating an instance of a model having a BooleanField my_boolean_field with a default set to True, I get an error:

my_boolean_field is required

Shouldn't it be set to the default value?

models.py

class MyModel(User):

    my_boolean_field = models.BooleanField(default=False)

admin.py

class MyModelCreationForm(UserCreationForm):   

    my_boolean_field = forms.BooleanField(initial=False)

    class Meta:
        model = User

class MyModelChangeForm(UserChangeForm):

    my_boolean_field = forms.BooleanField(initial=False)

    class Meta:
        model = User


class MyModelAdmin(UserAdmin):

    form = MyModelChangeForm
    add_form = MyModelCreationForm

    list_filter = ()
    list_display = ('username', 'my_boolean_field')

    fieldsets = (
        (None, {'fields': ('username', 'my_boolean_field', 'password' )}),
    )


    add_fieldsets = (
        (None, {
            'classes': ('wide',),
            'fields': ('username', 'my_boolean_field', 'password1', 'password2')}
        ),
    )

    def get_form(self, request, obj=None, **kwargs):
        form = super(MyModelAdmin, self).get_form(request, obj, **kwargs)
        if obj==None:
            form.base_fields['username'].widget.attrs['autocomplete'] = 'off'
            form.base_fields['password1'].widget.attrs['autocomplete'] = 'off'
            form.base_fields['password2'].widget.attrs['autocomplete'] = 'off'
        return form

samsic_site.register(MyModel, MyModelAdmin)