[racket] importing signatures

Here's the file "sig":

  #lang racket
  (define-signature Foo (make-foo foo?))

Here's another file:

  #lang racket/unit
  (require "sig")
  (import)
  (export Foo)
  (define-struct foo ())

When I execute this, I get

  define-unit: unknown signature in: Foo

I would have thought that the the signature is precisely what the
(require "sig") line would have imported.  I must be missing something
in the grammar -- anyone?

Shriram