[plt-scheme] "identifier originates in a different module"

I'm trying to define a macro in a module to make invoking units
easier, and I'm running into an error message that I don't understand.

(module foo mzscheme
  (require (lib "unit.ss"))
  (provide foo@ define-foo)
  (define foo@
    (unit (import number) (export foo)
      (define (foo) number)))
  (define-syntax define-foo
    (syntax-rules ()
      ((_ prefix number)
       (define-values/invoke-unit (foo) foo@ prefix number))))
)

(module use-foo mzscheme
  (require foo)
  (provide main)
  (define number 42)
  (define-foo x number)
  (define (main) (x:foo)))

(require use-foo)
(main)

foo.scm:10:36: module: identifier originates in a different module at: x:foo in: (define-values (x:foo) (invoke-unit (compound-unit (import number) (link (unit-to-invoke (foo@ nu...

Can anyone shed light on this?  What is wrong with an identifier
originating in a different module, and what should I do about it?

--dougo at ccs.neu.edu