validatecommand

How can I return a value from a validatecommand?
(defparameter *y* (make-instance 'entry :grid '(0 0) :validate :key))
(configure *y* :validatecommand #'(lambda (&rest args) (print args)
(print (text *y*)) t))
(mainloop)
When I change the entry, I get 'Error: expected boolean value but got "" ...'