[plt-scheme] unbound variable

Can someone tell me what I've done wrong? 

I wrote some scheme code that uses the Swindle clos library.  It works
fine when I run tests on it, but as soon as it was enclosed within a
module statement I got the error:

                "expand: unbound variable in module in: :accessor"

The code looks like this: 

(module validation mzscheme
      (require "collects/swindle/clos.ss")
  
      (defclass <validation-error> ()
          (message :accessor msg  :initarg :msg :initvalue "")
          (param   :accessor param   :initvalue "" :initarg :param) 
          (value   :accessor value   :initvalue "" :initarg :value) )
...etc....
)

I'm using the full swindle language option. 

Thanks in advance for your help.