my expression fails with a runtime exception

Question

I'm attempting to bring back one of the element values from an IDOC, to determine a variable value. When I run this in the debugger just to test it brings back the right value, but when i run it as part of the expression it fails -

import java.lang.String;

String destination = #[xpath3('*/IDOC/EDI_DC40/RCVPRN',payload,'STRING')];
    if(destination.equals('EUMULESOFT')) {
        flowVars.vDestination = "EuropeanDC";
    }
    else {
        flowVars.vDestination = "WestwayDC";
    }

Is it the MEL expression itself? I was wondering whether perhaps i needed payload.xpath3 instead?