Abblying absolute height to minibuffer

I want to change the absolute text height used by the minibuffer.
Am using a defcustom with a :set function.  And another function
to use with minibuffer-setup-hook.  But I cannot see the height 
changing correctly.

What could I be doing wrong?


(defcustom cork-height 140
  "Minibuffer face height set by `cork-height-psw'."
  :type '(number)
  :group 'cork
  :set (lambda (symbol value)
         (set symbol value)
         (when (facep 'minibuffer-prompt)
           (set-face-attribute 'minibuffer-prompt nil :height value))))

(defun cork-bait ()
  "Configure the minibuffer face height using a pre-set variable.  Designed
to work as a hook function for `minibuffer-setup-hook', so it takes no
arguments."

  ;; A face spec is an alist.
  (set (make-local-variable 'face-remapping-alist)
         `((default . (:height ,cork-height)))))