Sorry I mean after making sure it's a number, cast to a String. I've had to do this before - some places in NS won't let you use an actual number for something that should be numeric, it drives me crazy. An example is trying to return a field of type List/Record from a custom Workflow action to a Workflow field (also of type List/Record) that doesn't have "store value" ticked. You get a similar error unless you wrap your return value in String().