Your are probably trying to evaluate a non-defined field that has a null value as though it is text. The Blue Prism calculation stage has no concept of null (so there is no IsNull function available - which is an oversight worth raising in the ideas & enhancements area of this forum) - but instead of a proactive check you could have a retrospective check by catching the exception and checking for that left-hand value empty error - if that error occurs take your flow to the same rout as your field """" decision.
