Technically the separator is ", " (comma and space), so maybe try that. Obviously this doesn't solve the problem if your value contains that same string. Generally speaking, when you drop on args.Line it becomes hack-territory, so chances that things will break somewhere are high.
An alternative approach could be to map that extra field to a spare UD (if you have it), so you can reference it in a Conditional Rule from the transformation rule of the dimension you're writing the expression for; all the UD values could then transform to None with a wildcard mask.
