CiviReport: Contribution Summary - groupBy split personality issue

Under PHP 7.1, when running the Contribution Summary report I received:

Error: [] operator not supported for strings in CRM_Report_Form_Contribute_Summary->groupBy() (line 580 of /home/anthony/buildkit/build/devtraining/sites/all/modules/civicrm/CRM/Report/Form/Contribute/Summary.php).

In looking at the groupBy() function, it is initialized as a string but later assigned as if it were an array in:

$this->_groupBy[] = self::fiscalYearOffset($field['dbAlias']);

later on it is deconstructed and reconstructed:

 $groupBy = array();
      foreach ($this->_groupBy as $key => $val) {
        if (strpos($val, ';;') !== FALSE) {
          $groupBy = array_merge($groupBy, explode(';;', $val));
        }
        else {
          $groupBy[] = $this->_groupBy[$key];
        }
      }

Can someone work to rework this so that it is less confusing and more consistent so that it will work in PHP 7.1.