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

[Eli Zaretskii <eliz@gnu.org>]

> From: Milan Zamazal <pdm@zamazal.org>
> Date: Thu, 12 May 2016 21:35:55 +0200
> 
> 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

Do you (or someone else) understand why Czech and Slovak input methods
include kp-* keys in the input method definitions?  Like this:

(quail-define-rules
 ("1" ?+)
 ...
 ("=}" ?})
 ([kp-1] ?1)
 ([kp-2] ?2)
 ([kp-3] ?3)
 ([kp-4] ?4)
 ([kp-5] ?5)
 ([kp-6] ?6)
 ([kp-7] ?7)
 ([kp-8] ?8)
 ([kp-9] ?9)
 ([kp-0] ?0)
 ([kp-add] ?+))

What does this try to accomplish, and why?  No other input method does
that, and the bindings this tries to produce are already available by
default, so what is their purpose?  Those bindings are the cause of
this problem, AFAICT.

Thanks.