Steam Can't Bind Action To Ctrl Key On Its Own

I'd like to bind crouching to Ctrl on its own, as I did in previous TSW games. However, TSW5 seems to only allow you to bind Ctrl to an action in combination with another key.

 

CTRL is already used as a modifier for many controls in the game, it may be why it doesn't let you

 

Yes, you cannot bind a key used as a qualifier to an action itself. (Toggle AFB / Confirm AFBVSoll)