[plt-scheme] Signatures, am I missing something?

Hello all,

I've used a-signature.ss and a-unit.ss a lot by now but right now I'm
getting an error I cannot even begin to understand and probably is
because I'm missing something really stupid but I can't find what:

cnfreader-sig.ss:
(module cnfreader-sig (lib "a-signature.ss")
  get-clauses-from-cnf)

database.ss:
(module database (lib "a-unit.ss")

  (require  "cnfreader-sig.ss")

  (import cnfreader-sig^)
  (export)
  (void))

I'm getting:
define-unit: unknown signature in: cnfreader-sig^

But I can't understand why, I have this pattern of usage in other
files and they work. :-/
[version info DrScheme, version 369.8-svn6apr2007 [3m].]

Any ideas?

Cheers,
-- 
Paulo Jorge Matos - pocm at soton.ac.uk
http://www.personal.soton.ac.uk/pocm
PhD Student @ ECS
University of Southampton, UK