drupal 7 states, set disabled unless any checkbox in a group is checked
<p>I currently have a form with a submit button that I would like to leave disabled unless any checkbox(es) in a group of them is checked. Currently, no matter what I do the button is not disabled. Here's the code I'm using:</p>

<pre><code>$form['enrollments']['submit'] = array(
      '#type' =&gt; 'submit',
      '#value' =&gt; t('Submit Change'),
      '#states' =&gt; array(
      'disabled' =&gt; array(
           'input:checked' =&gt; array('length' =&gt; 0),
      ),
 );
</code></pre>

<p>The checkboxes all share a shame name in array form like "students[12345]". So, if any one of those "students" checkboxes is checked the button should be enabled. However, given the code above the button is always enabled.</p>