mapping Message ID when more one source message

Hello,

I've a message mapping with two source message and one target message.

Also i've use a UDF to get the message ID. This UDF  fails. If i modified the mapping and delete one source message the UDF works OK (only when exist one unique source message).

Why?, How can i do it?

The code of UDF is:

String MessageID;

java.util.Map param = container.getTransformationParameters();

MessageID = (String) param.get (StreamTransformationConstants.MESSAGE_ID);

return MessageID;

Regards,

Silvia