Cannot aggregate on taxonomy term

I have a view of nodes that only shows two fields: the taxonomy terms from a selected vocabulary (1 out of 3 available vocabularies) and an integer. I wish to get a sum of the integers grouped by taxonomy term. When I do that, I get this error instead of a graph:
user warning: Unknown column 'unknown' in 'order clause' query: SELECT ROUND(COUNT(node_data_field_hours.field_hours_value), 2) AS field_hours_value, term_node.tid AS term_node_tid FROM issuetracker_node node LEFT JOIN issuetracker_content_type_time_track node_data_field_hours ON node.vid = node_data_field_hours.vid LEFT JOIN issuetracker_content_type_time_track node_data_field_hours2 ON node.vid = node_data_field_hours2.vid LEFT JOIN issuetracker_term_node term_node ON node.vid = term_node.vid WHERE node.type in ('time_track') GROUP BY term_node.tid ORDER BY unknown ASC in /var/www/drupal/sites/issuetracker/modules/views/includes/view.inc on line 759.

Here's the view export:

$view = new view;
$view->name = 'chart';
$view->description = '';
$view->tag = '';
$view->view_php = '';
$view->base_table = 'node';
$view->is_cacheable = FALSE;
$view->api_version = 2;
$view->disabled = FALSE; /* Edit this to true to make a default view disabled initially */
$handler = $view->new_display('default', 'Defaults', 'default');
$handler->override_option('fields', array(
  'field_hours_value' => array(
    'label' => 'Hours',
    'alter' => array(
      'alter_text' => 0,
      'text' => '',
      'make_link' => 0,
      'path' => '',
      'link_class' => '',
      'alt' => '',
      'prefix' => '',
      'suffix' => '',
      'help' => '',
      'trim' => 0,
      'max_length' => '',
      'word_boundary' => 1,
      'ellipsis' => 1,
      'strip_tags' => 0,
      'html' => 0,
    ),
    'link_to_node' => 0,
    'label_type' => 'widget',
    'format' => 'default',
    'multiple' => array(
      'group' => TRUE,
      'multiple_number' => '',
      'multiple_from' => '',
      'multiple_reversed' => FALSE,
    ),
    'exclude' => 0,
    'id' => 'field_hours_value',
    'table' => 'node_data_field_hours',
    'field' => 'field_hours_value',
    'relationship' => 'none',
  ),
  'tid' => array(
    'label' => 'All terms',
    'alter' => array(
      'alter_text' => 0,
      'text' => '',
      'make_link' => 0,
      'path' => '',
      'link_class' => '',
      'alt' => '',
      'prefix' => '',
      'suffix' => '',
      'help' => '',
      'trim' => 0,
      'max_length' => '',
      'word_boundary' => 1,
      'ellipsis' => 1,
      'strip_tags' => 0,
      'html' => 0,
    ),
    'type' => 'separator',
    'separator' => ', ',
    'empty' => '',
    'link_to_taxonomy' => 1,
    'limit' => 1,
    'vids' => array(
      '9' => 9,
      '5' => 0,
      '7' => 0,
      '8' => 0,
    ),
    'exclude' => 0,
    'id' => 'tid',
    'table' => 'term_node',
    'field' => 'tid',
    'relationship' => 'none',
  ),
));
$handler->override_option('filters', array(
  'type' => array(
    'operator' => 'in',
    'value' => array(
      'time_track' => 'time_track',
    ),
    'group' => '0',
    'exposed' => FALSE,
    'expose' => array(
      'operator' => FALSE,
      'label' => '',
    ),
    'id' => 'type',
    'table' => 'node',
    'field' => 'type',
    'relationship' => 'none',
  ),
));
$handler->override_option('access', array(
  'type' => 'none',
));
$handler->override_option('cache', array(
  'type' => 'none',
));
$handler->override_option('items_per_page', 0);
$handler->override_option('style_plugin', 'chart');
$handler->override_option('style_options', array(
  'grouping' => '',
  'charts' => array(
    '#plugin' => 'google',
    '#type' => 'pie2D',
    '#width' => '400',
    '#height' => '200',
    '#color_palette' => '#ffffff,#000000,#ff0000,#00cc00,#0066b3,#ff8000,#ffcc00,#330099,#990099,#ccff00',
  ),
  'show_legend' => 1,
  'show_sums' => 1,
  'aggregation_field' => 'tid',
  'calc_fields' => 'field_hours_value',
  'calc' => 'COUNT',
  'precision' => '2',
));
$handler = $view->new_display('page', 'Page', 'page_1');
$handler->override_option('path', 'chart');
$handler->override_option('menu', array(
  'type' => 'none',
  'title' => '',
  'description' => '',
  'weight' => 0,
  'name' => 'navigation',
));
$handler->override_option('tab_options', array(
  'type' => 'none',
  'title' => '',
  'description' => '',
  'weight' => 0,
));