Thanks for your feedback.

>
> Thanks.  Hmm - one kind of raw prefix arg you don't handle yet is `-` 
> which is synonymous for -1.  M-- C-x 5 u gives the not so nice error 
> message:
>
> Wrong type argument: number-or-marker-p, -
>

Indeed, I forgot to handle that case.

>
> Oh, and, I think all explicit `error' calls in your patch could be made 
> `user-error's instead, WDYT?
>

That makes sense, indeed.  Done.

Updated patch attached.