Remote invoke gives null java object

I am doing a remote invoke using a java client on a flow service,
which is supposed to insert a custom java object into the pipeline
and IData is returned.
The java object is in the pipeline (I can see the non-null value in
serevr log), but the java client sees null. Other string variables
are available in the IData, but this java object in question is null.

the client JVM version 1.3
the WM version is 6.1 and the WM JVM version is 1.4.2.

Any help is appreciated.