Delete key on dropdown relationship field returns error

I came across this issue completely on accident today.

I have a custom post type (tournament) that is connected to another custom post type (tour) by a tour_id field. This displays in the tournament admin page as a dropdown field. When on that field, if you hit the delete key on a blank field, the field changes to “There was an error rendering the field.”.

Note that if you start typing into the field and hit delete too many times, this also happens.

This is happening in Firefox and Safari.

Mike