[plt-scheme] expand

I can't figure out why the following gives an error.  Can someone help?

(let-syntax ((foo (lambda (obj)
                    (syntax-case obj ()
                      ((_ q) (expand (syntax q)))))))
  (let ((y 3)) (foo y)))

It seems that expand somehow makes the macro system lose track of the
origin of y, but I don't see why.

David