Why does this code sometimes work?

[jacksneckhurts]

I pulled the following code from a tutorial:

;; begin
(defun multiply-by-seven (number)
  "Multiply NUMBER by seven."
  (* 7 number))

(multiply-by-seven 3)
;; end

I put it in emacs as is, and sometimes it runs, and most of the time it
errors out... I run the code with "ctrl-x ctrl-e" while the cursor is
to the right of "3)". What am I doing wrong? Thanks!