[plt-scheme] equal strings aren't equal

I'm using mime-get-bindings to get bindings from a multipart form.  In
that form there is a field

<input name="act" type="hidden" value="doit">

I then have some code like this:

(let* ([b (mime-get-bindings)]
        [act (extract-binding/single "act" b)])
  (cond
     [(string=? act "view") ...]
     [(string=? act "doit") ...]
     [else *generate-error-page*]))

except that the test always fails for "doit", but always works for
"view".  Anybody know why?

-Andrey