PayPal Standard IPN fails with "Invalid input parameters"

We started to see PayPal Standard IPN notifications fail, with the error log reporting:

PHP Fatal error: Uncaught CRM_Core_Exception: [0: Invalid input parameters\n\n thrown in /PATH/administrator/components/com_civicrm/civicrm/CRM/Core/Payment/BaseIPN.php on line 76

Tracing the problem, I found the issue was with CRM/Core/Payment/PayPalIPN.php, line 55. At that point, we merge two arrays and pass to setInputParameters. The problem is that if the second array was empty, it was overwriting the entire array and passing an empty value to that method.