Error in script (Global) function <reveal>: chunk_3 (426, 6-54): bad argument to #2 to 'find' (string expected, got nil)

Happens when it tries to draw the monster cards. It wont no matter which monster are laid out. Error appears all times, across loads and scenarios.
Monster maths are placed using the Game Setup - Monster setup interface

If we press "draw" on monster mats manually it will visualize correctly in the initiative list.