bug#23559: 25.0.93; Wrong type argument on C-u C-x = with Czech input method

[Milan Zamazal]

When I press `C-u C-x =' with a Czech input method enabled, I get the
following error:

  quail-gen-decode-map: Wrong type argument: characterp, kp-add

How to reproduce the bug:

- Start Emacs, e.g.: emacs -Q
- Select Czech input method: C-x RET C-\ czech RET
- Make sure you are on some character in the buffer, e.g. go to the
  beginning of *scratch* buffer with the semicolon character.
- Run what-cursor-position with a prefix argument: C-u C-x =
- You should receive the error above.

The bug is also present in Emacs 24.5.