Array to string conversion error

I get this error:
Notice: Array to string conversion in dxfc_century_club_block_view() (line 83 of /var/www/html/sites/all/modules/dxfc_century_club/dxfc_century_club.module).
...with this code:

function dxfc_century_club_block_view($delta = '') {
  $block = '';
  switch ($delta) {
    case 'dxfc_century_club':
      $block['subject'] = t('DXFC Century Club');
      if (user_access('access content')) {
        // Use our custom function to retrieve data.
        $result = dxfc_century_club_contents('block');
        // Declare an array to contain items for the block to render.
        $items = array();
        // Iterate over the resultset and cleanse.
        foreach ($result as $entry) {
          $items[] = array_map('check_plain', (array) $entry); 
        }
       // No content
        if (empty($items)) {
          $block['content'] = t('No records available or module failure.');
        }
        else {
          // Pass data through theme function.
          $header = array(t('Callsign'), t('DXFC Count'));
//LINE 83...
          $block .= theme('table', array('header' => $header, 'rows' => $items));
        }
      }
    return $block;
  }  
}

Can someone please explain why I get this error from this line?

I'm so sorry I've got to the point where I've had to ask this!

Stewart