Pagemediaselect: Error when saving

I wanted to extend the blueprint of a page by adding a pagemediaselect field, which works perfectly when it's set to "single item".

header.images:
    type: pagemediaselect
    label: Images

But when I add multiple: true the page can't be saved anymore. Whoops says it is due to an Array to string conversion error, after trying to save the page with some images selected.
I also checked this behavior by adding an selectize field to the blueprint (which I think should theoretically work similar), but it worked flawlessly.

I had the same problem when using multiple pages field. I was able to solve this by adding validate.type:

header.pages:
   type: pages
   label: Pages
   multiple: true
   validate:
       type: array

It might help you or point you to the right direction.