[plt-scheme] fred doesn't respond to text-field changes

hi,
  the mred binding to gui doesn't seem to be responding to text-field
changes, is this a work in progress or am i missing something in my
program?

#lang frtime
(require frtime/gui/fred )
(define a (new ft-frame% (label "hello") (width 600) (height 500)
(shown #t)))
(define b (new ft-text-field% (label "hello") (parent a)))
(define k (send b get-value-b))
(define c (new ft-button% (label k) (parent a)))

(value-now k) indicates that k changes with text-field changes but the
button c doesn't update to reflect this, (replacing k with a behaviour
like (number->string seconds) works fine.)
harsha