POJO Transaction Splitting getter chain parameters

I am trying to split transactions based on specific content in a method's string parameter, but can't seem to get the getter chains to work the way I want it to.

The entry method signature is like this: invoke(String x).  The content of x is typically something like this: <msg>NEW</msg>

If I configure the Getter Chain to be toString() then the string correctly appends to the Transaction name (i.e. TXN-<msg>NEW</msg>)

But if I configure the Getter Chain to be toString().indexOf("NEW"), it ends up always returning -1 which means it's not found.  I am expecting it to return 5 so that the transaction name resolves to TXN-5.  

Is there a way this can be done or am I misusing getter chains here?