[plt-scheme] unit/sig: not a signature at: drscheme:tool-exports^ ?

Hi folks,

I'm writing a tool and got an error.

setup-plt:   in c:\documents and settings\grettke\application data\plt scheme\37
0\collects\drsyncnsave\
tool.ss:12:14: unit/sig: not a signature at: drscheme:tool-exports^ in: (unit/si
g drscheme:tool-exports^ (import drscheme:tool^) (define (phase1) (message-box "
tool example" "phase1")) (define (phase2) (message-box "tool example" "phase2"))
)

Here is the file itself:

; $LastChangedDate: 2007-05-23 07:53:36 -0500 (Wed, 23 May 2007) $
; $LastChangedRevision: 46 $
; $HeadURL: svn://osiris/drsyncnsave/trunk/tool.ss $
(module tool mzscheme
  (require (lib "tool.ss" "drscheme")
           (lib "mred.ss" "mred")
           (lib "unitsig.ss"))

  (provide tool@)

  (define tool@
    (unit/sig drscheme:tool-exports^
      (import drscheme:tool^)
      (define (phase1)
        (message-box "tool example" "phase1"))
      (define (phase2)
        (message-box "tool example" "phase2")))))

This is strange. I looked at a few other tools and this one sure looks correct!