According to the docstring of define-key, a cons of (MAP . CHAR) is a valid
key definition that will act as CHAR in MAP. This worked in previous
versions of Emacs but now causes an error when the keybinding is executed:
"Wrong type argument: commandp..."

Initial discussion took place here:
https://emacs.stackexchange.com/questions/36641/binding-a-key-to-map-char