Django Admin Fieldsets is not working

[almehady]

class DemoAdmin(admin.ModelAdmin):
  list_display = ('name', 'city')
  fieldsets = (
      ('Standard info', {
          'fields': ('name')
      }),
      ('Address info', {
          'fields': ('address', ('city', 'zip'))
      }),
   )

Changing Fieldsets does not do any effect in admin form.