Populating domain attribute into FIM. The return type(Object) of function IIF is not Binary

I'm trying to use the Function "ConvertSidToString" with the CustomExpression:

IIF(Eq(Left(ConvertSidToString(objectSid),41),"S-1-5-21-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx"),"DOMAINA",IIF(Eq(Left(ConvertSidToString(objectSid),41),"S-1-5-21-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx"),"DOMAINB",IIF(Eq(Left(ConvertSidToString(objectSid),41),"S-1-5-21-xxxxxxxxxx-xxxxxxxxxx-xxxxxxxxxx"),"DOMAINC","Unknown")))

However, I get the error: The return type(Object) of function IIF is not Binary