Upgrading v.5 to v.6 - Views - Field handler event.event_start is not available

Can anyone advise how I get round this error message?

Have taken a straight export from version 5 and am importing into version 6.
All the other views so far have imported with no problems and display fine.

This particular view is based on content created using the 'events' module and no problems at all in version 5.

  $view = new stdClass();
  $view->name = 'TrainingStage1';
  $view->description = 'A list of training dates for Stage 1';
  $view->access = array (
);
  $view->view_args_php = '';
  $view->page = TRUE;
  $view->page_title = 'Stage 1 training dates';
  $view->page_header = '';
  $view->page_header_format = '1';
  $view->page_footer = '';
  $view->page_footer_format = '3';
  $view->page_empty = '';
  $view->page_empty_format = '3';
  $view->page_type = 'table';
  $view->url = 'view/stage1';
  $view->use_pager = TRUE;
  $view->nodes_per_page = '25';
  $view->block = TRUE;
  $view->block_title = 'Stage 1 training days';
  $view->block_header = '';
  $view->block_header_format = '1';
  $view->block_footer = '';
  $view->block_footer_format = '1';
  $view->block_empty = '';
  $view->block_empty_format = '1';
  $view->block_type = 'table';
  $view->nodes_per_block = '25';
  $view->block_more = FALSE;
  $view->block_use_page_header = FALSE;
  $view->block_use_page_footer = FALSE;
  $view->block_use_page_empty = FALSE;
  $view->sort = array (
    array (
      'tablename' => 'event',
      'field' => 'event_start',
      'sortorder' => 'ASC',
      'options' => '',
    ),
  );
  $view->argument = array (
  );
  $view->field = array (
    array (
      'tablename' => 'node',
      'field' => 'title',
      'label' => '',
      'handler' => 'views_handler_field_nodelink',
      'options' => 'link',
    ),
    array (
      'tablename' => 'event',
      'field' => 'event_start',
      'label' => '',
      'handler' => 'views_handler_field_date_small',
      'sortable' => '1',
    ),
  );
  $view->filter = array (
    array (
      'tablename' => 'term_node',
      'field' => 'tid',
      'operator' => 'OR',
      'options' => '',
      'value' => array (
  0 => '245',
),
    ),
    array (
      'tablename' => 'node',
      'field' => 'status',
      'operator' => '=',
      'options' => '',
      'value' => '1',
    ),
  );
  $view->exposed_filter = array (
  );
  $view->requires = array(event, node, term_node);
  $views[$view->name] = $view;