Django admin breaking with non-default primary_key for model with a m2m relationship ?

Question

I have a simple Post model with a m2m field to a Tag model.

The Tag had for some reason to use a non default primary key.

Inside the admin page for a Post, the labels for the multiple selection field for Tags appear, but not the input field itself. I also tried using the filter_horizontal for the tags, but still only the labels appear without the actual field.

Any ideas why it breaks and/or workarounds?

Thanks!