Woo, good to see that you able to figure it out alone 
It still is a bug since the error you got should never happen it’s actully place there to prevent potential instybilities from unpredicted behaviors. It means there errorus combination of nodes that blueprint designer don’t prevent it and/or compiler don’t properly inform user that it is wrong, insted it hits that ensure function giving cryptic error which you need to debug engine to understand. Now this will most likely be fix.