"Cannot POST /dialog/authorize/decision" while implementing the OAuthorize example

I'm implementing OAuth 1.0a with the OAuthorize library. I'm following the example provided by the library.

I'm hosting my app at Heroku. Using the test client I'm able to GET a request token, and use it to /dialog/authorize:

Hi Bob Smith!

Samplr is requesting access to your account.

Do you approve?

When I press Allow, I get a Cannot POST /dialog/authorize/decision.

I've registered the endpoint in app.js:

app.post('/dialog/authorize/decision', oauth.userDecision);

Either I am missing something silly from the OAuthorize example or there is a fault in the way I'm handling the transaction.

dialog.ejs:

<form action="/dialog/authorize/decision" method="post">
  <input name="transaction_id" type="hidden" value="<%= transactionID %>">
  <div>
    <input type="submit" value="Allow" id="allow">
    <input type="submit" value="Deny" name="cancel" id="deny">
  </div>
</form>