Enable Fixed Expiration Date causes ERROR

I have tried to use the Advance setting Enable Fixed Expiration Date in my plan, filling out both the expiration date and the dates between which the assignment occurs. When attempting to assign this plan via the backend I receive

ERROR Call to a member function toUnix() on bool
/home/alkhamsa/public_html/administrator/components/com_payplans/includes/event/events/core.php:389

If I disable that option the plan assigns successfully. Is this a known problem? Couldn't find it in the forums. Have PayPlans 4.2.6 on Joomla 3.9.25.