User registration form field displays when user field is locked accompanied by error message.

I received the following error when I went to http://example.com/admin/config/people/accounts/fields/example_field

Notice: Undefined index: instance in user_form_field_ui_field_edit_form_alter() (line 3644 of /modules/user/user.module).

The reason I received this error was because I pragmatically created a field for a user where locked is TRUE

/*
 * Returns the fields
 */
function example_fields() {
	return array(
		array(
			'field_name' => 'example_field',
			'type' => 'text',
			'locked' => TRUE,
			'settings' => array(
				'no_ui' => TRUE,
				'max_length' => 100,
			),
		),
	);
}

/*
 * Returns the field instances
 */
function example_instances() {
	return array(
  	array(
  		'field_name' => 'example_field',
  		'entity_type' => 'user',
  		'bundle' => 'user',
   		'label' => 'Example Field',
   		'widget' => array(
  			'settings' => array(
					'size' => 100,
				),
			),
  	),
  );
}

/**
 * Implements hook_install().
 */
function example_install() {

	// Create the subscriber fields
	foreach (example_fields() as $field) {
		field_create_field($field);
	}

  foreach (example_instances() as $instance) {
  	field_create_instance($instance);
  }
  
}

/**
 * Implements hook_uninstall().
 */
function example_uninstall() {

	foreach (example_instances() as $instance) {
		field_delete_instance($instance);
	}
}

If I change locked to FALSE then I stop receiving the error.