about register

[Le TeXnicien de Surface]

hello

I'd like to use the following command:
(defun complete-pid ()
  (interactive)
  (goto-char 0)
  (number-to-register 1 "A")
  (while (re-search-forward "p=PID=0" nil t)
    (insert-register "A")
    (increment-register 1 "A")
))

but when emacs finds the first `p=PID=' it says:
`register does not contain a number'
and stops.

it did work yesterday and I've not changed anything since (perhaps I did
something before using the command but I can't remember what)

Any hint appreciated

thanks for your reading so far

-- 
Le TeXnicien de Surface