title: Emacs hotkey to align equal signs
body: <p>I'd like to put something like this in my .emacs:</p>
  
  <pre><code>(local-set-key (kbd "C-c a =") 
    (lambda () (interactive) 
      (align-regexp (region-beginning) (region-end) "=")))
  </code></pre>
  
  <p>But whenever I run it, I get an error "Wrong type argument: numberp, nil".</p>
  
  <p>What does this error mean and how do I get the effect I'm looking for?</p>