Error Undefined before Show message

I have a microflow that checks if a combination of 2 attributes already exists because I want that combination to be unique.  When the combination exists the user gets the message that it already exists.  This microflow is set in an before commit event handler of the entity and the option 'Raise an errror when the microflow returns false' is not checked.  When I add a new record with a combination that already exists, I get the message that it already exists. BUT, before that I get an error undefined message. Any ideas how to fix this?   I have made the following microflow:  And the error: 
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