Numbered regexps throw invalid regex error

[Tom]

According to the docs:

  This construct allows you to force a particular
  group number.  There is no particular restriction on the numbering,

but when I try this:

  (looking-at "\\(a\\(?1:b\\)\\)")

then it throws an error.

I know it's possibly a collision with the containing group's number
and I can overcome it by using a larger number, but the doc says
I can use it to force a group number, so why does it throw an error
then?

Is it a bug? I tried it with GNU Emacs 23.2.1