emacs swaps buffer between two windows

  I wrote this code yesterday evening. It swaps buffer between two windows in emacs.
It doesn't work that the number of window is not equals TWO.


[code]
(defun qw-swap-window-split ()
  "This code swap two buffers in  two windows, arranged left-right or
up-down. It  works for one frame of two windows."
  (interactive)
  (if (= (count-windows) 2)
      (let* ((cur-buf (window-buffer))
             (next-buf (window-buffer (next-window))))
        (set-window-buffer (selected-window) next-buf)
        (set-window-buffer (next-window) cur-buf))))
[/code]