lisp on emacs: how to comment a multi-line expression?

For example, if I want to comment this:

(defun noop ()
  nil)

Every time I try to put a semicolon before the "(defun", the defun runs away to the next line. So how is that supposed to be done?

GNU Emacs 23.1.1

Edit: by "running away" I mean when I insert a semicolon before "(defun", a newline is automatically inserted after the semicolon and "(defun" starts on a new line again.