Sync rules saying '' is not a valid attribute when custom expression contains a string with comma

Hi all,

I feel like I must be missing something obvious, but I have a CustomExpression sync rule that looks like this:

IIF(IsPresent(staff_title),UpperCase(staff_surname)+" "+staff_knownas+" ("+staff_title+")",UpperCase(staff_surname)+", "+staff_knownas)

FIM won't let me save it - it says '' is not a valid attribute.

If I change the rule to:

IIF(IsPresent(staff_title),UpperCase(staff_surname)+" "+staff_knownas+" ("+staff_title+")",UpperCase(staff_surname)+" "+staff_knownas)

It works fine.  I've tried lots of other characters; full stops, hyphens all work fine.  But the comma breaks it.  A colleague suggested flowing statically the comma to every user as an attribute and referring to the attribute name, but surely that's not a good workaround.

Please tell me I'm doing something stupid!

Thanks,

Paul.