Zombie.js can't find button element on page

I'm trying to submit a form with zombie.js, and pressButton() is failing with "Error: No BUTTON 'xxx'", where xxx is the selector. Here is the app:

var Browser = require('zombie');
b = new Browser();
b.visit('https://www.example.com/login/', function() {
        b.
          fill('name', 'My Name').
          fill('code', 'My Code').
          pressButton('submit', function() {
                console.log(b.html());
          });
});

And here is the form:

<form method="post" class="login">
    <p> <label for="name"> <span id="nameprompt">Your Name:</span> </label> </p>
        <input name="name" id="name" value="" size="40" maxlength="40" />
    <p> <label for="code"> <span id="codeprompt">Bar Code</span> </label> </p>
        <input name="code" id="code" type="PASSWORD" value="" size="40" maxlength="40" />
    <div class="formButtonArea">
        <input type="image" src="/screens/pat_submit.gif" border="0" name="submit" value="submit" />
    </div>
</form>

I've tried a bunch of selectors including things like ".formButtonArea input" in addition to the obvious "submit" with no success.

What am I doing wrong?