missing single quotes around arrayed variable reference

On returning to the website after the PayPal transaction (PayPal Standard) for a recurring membership where the membership is attached to an Organization, the payment and membership creation complete successfully, but the following error is shown to the user:
Notice: Use of undefined constant contributionID - assumed 'contributionID' in CRM_Core_Payment_PayPalImpl->doTransferCheckout() (line 513 of /home/musicnova/www/www/sites/all/modules/civicrm/CRM/Core/Payment/PayPalImpl.php).

The line is question is:
$cancelUrlString .= "&isRecur=1&recurId={$params['contributionRecurID']}&contribId={$params[contributionID]}";

which certainly looks like missing single-quotes around contributionID. Adding them resolves the issue. I experienced it under CiviCRM 4.2.7 but the missing single quotes persist in the equivalent 4.3.0 PayPalImpl.php file.