Extend custom page blueprint

I have created a theme with custom blueprints to configure options to be shown in the Admin. This part works fine.
The problem is that I am unable to extend these blueprints. When I try to define a common blueprint and use it on multiple pages (these are root pages), I get this error:

“Argument 1 passed to Grav\Common\Page\Page::move() must be an instance of Grav\Common\Page\Page, null given, called in /vagrant/acade.local/public/user/plugins/admin/classes/admincontroller .php on line 498”

I have seen this post, but I still don’t get how to set this supposed missing variable:

Edit: Sorry, actually the problem is not the inheritance, but simply modular blueprints.

Ok I think I’ve figured it out, I added this hidden input to the modular blueprint:

type: hidden
default: ‘/’

I don’t know if it is the best solution, but it works.