JDBC and LO - invalid large object descriptor

Hi,

I have got a problem with reading large objects from Postgres through JDBC
(both 7.0.2):
a call to resultset.getBytes(...) results in:

FastPath call returned ERROR:  lo_tell: invalid large object descriptor (0)
at org.postgresql.fastpath.Fastpath.fastpath(Fastpath.java:141)
at org.postgresql.fastpath.Fastpath.fastpath(Fastpath.java:191)
at org.postgresql.fastpath.Fastpath.getInteger(Fastpath.java:203)
at org.postgresql.largeobject.LargeObject.tell(LargeObject.java:232)
at org.postgresql.largeobject.LargeObject.size(LargeObject.java:247)
at org.postgresql.jdbc2.ResultSet.getBytes(ResultSet.java:370)

using "select lo_export(...)" is no problem, so I suppose the data is stored
correctly within the DB.
Any idea where the 0 is comming from or what the problem could be?

ByeAndreas