Eli Zaretskii <eliz@gnu.org> writes:

> How would "make clean" help?  I did remove register.elc, it didn't
> help.

Not only register is involved here but indirectly cl-generic.

> OK, so I guess I shouldn't have tried to install it.

Here a patch with the change suggested by Stefan applied, slighly
modified though because it fails if I put the call to
cl--generic-prefill-dispatchers at the recommended place.

I recompiled Emacs with this patch with no errors.
 
-- 
Thierry