Fatal error: Cannot unset string offsets

theme_datatable() assumes that all the items in the $header array have datatable options:

    // or in each header definition.
    if (!isset($datatable_options['aoColumns'])) {
      foreach ($header as $key => $cell) {
        $datatable_options['aoColumns'][] = $cell['datatable_options'];
        unset($header[$key]['datatable_options']);
      }
    }

This causes a fatal if they are not:

( ! ) Fatal error: Cannot unset string offsets in sites/all/modules/contrib/datatables/datatables.module on line 166

Note that the docs say that this should work fine:

* @param $header
* An array containing the table headers. Each element of the array can be
* either a localized string or an associative array with the following keys:
* - "data": The localized title of the table column.
* - Any HTML attributes, such as "colspan", to apply to the column header
* cell.
* - "datatable_options": An associative array containing DataTable column
* specific features/options.