Sławomir Grochowski <slawomir.grochowski@gmail.com> writes:

>> Do not forget to add :package-version. 
>
> Thank you for bringing it to my attention.
>
> Patch in attachment.

Thanks!
I improved the patch a bit, with minor changes to the commit message and
adding more to the docstring and type spec of the defcustom. I also
changed the custom group (:group 'checkbox does not exist).

See the attached.