user/autocomplete not working

Fields that make use of 'user/autocomplete' as their #autocomplete_path no longer work in 7.39.

This bug in contrib might offer some clues: https://www.drupal.org/node/2556749

In firebug I am seeing "TypeError: $input[0] is undefined" when the page containing my form first loads.

I'm seeing this in a custom module whose autocomplete fields worked fine for a year or so prior to the 7.39 release. Here is an example of a form field that no longer works...

$form['student'] = array(
      '#title' => t('Student'),
      '#type' => 'textfield',
      '#size' => '20',
      '#autocomplete_path' => 'user/autocomplete',
      '#attributes' => array('id' => 'tutoring-student', 'placeholder' => t('Username')),
      '#required' => TRUE,
      '#default_value' => isset($tutoring->student) ? $tutoring->student : NULL,
      '#weight' => -8,
    );