[plt-scheme] match not working under plai

Using the PLAI - Advanced Student language, the following program:

  (require (lib "match.ss"))

  (match '(1 2)
    [`(,a ,b) (+ a b)])

gives the error:

  match: no matching clause for (list 1 2): (plt:match (quote (1 2)) ((quasiquote ((unquote a) (unquote b))) (+ a b)))

Under other languages, the program correctly produces 3.  Any idea
what's wrong?  Thanks.

Dave