Help - lo_close: invalid large obj descriptor

I've read the docs, I've searched the web.

I'm running PGSql 7.3.4 under Cygwin 1.3.22.1, using the latest JDBC v3
drivers (jar file).
Accessing the database works fine, but when I try to write to a newly
created LargeObject it always fails with

org.postgresql.util.PSQLException - FastPath call returned
ERROR: lo_write: invalid large obj descriptor (0)

The code below illustrates this.

The code is actually going to be used within a Tomcat webapp, but I had to
use Jdbc3ConnectionPool since that was the only pool that returned
connections that were castable to a PGConnection. I get the same error from
inside and outside Tomcat. Thought the error might be related to
permissions writing large objects so I tried the following which didn't help.

GRANT ALL ON "pg_largeobject" TO "web";

So I suspect the problem is related to using a pooled connection. Can
someone shed some light?

import java.io.File;
import java.io.FileInputStream;
import java.sql.Connection;
import java.sql.SQLException;

import org.postgresql.PGConnection;
import org.postgresql.jdbc3.Jdbc3ConnectionPool;
import org.postgresql.largeobject.LargeObject;
import org.postgresql.largeobject.LargeObjectManager;

public class TestHarness
{
public static void main(String[] args)
{
LargeObject lo = null;
Connection conn = null;
try
{
//get a connection that's PGConnection-castable.
//PoolingDataSource- tried 'n died
//Jdbc3PoolingDataSource- tried 'n died
Jdbc3ConnectionPool source = new Jdbc3ConnectionPool();
source.setServerName("ash");
source.setDatabaseName("adserver");
source.setUser("web");
source.setPassword("web");
source.setDefaultAutoCommit(false);

//just to be sure - it always says true so that's good
conn = source.getConnection();
System.out.println(conn.getAutoCommit());

//this only works for connections from Jdbc3ConnectionPool
PGConnection pgconn = (PGConnection)conn;

//create the largeobject and prepare to write to it
LargeObjectManager lom = pgconn.getLargeObjectAPI();
int oid = lom.create(LargeObjectManager.READ |
LargeObjectManager.WRITE);
lo = lom.open(oid, LargeObjectManager.WRITE);

//the thing to write
File file = new File("D:\\downloads\\images\\banner.GIF");
FileInputStream fis = new FileInputStream(file);

//prepare a buffer (small enough to force multiple loop iterations
//so we test it properly)
byte buff[] = new byte[1024];

//now write the image
int bytes_read;
while ((bytes_read = fis.read(buff, 0, buff.length)) > 0)
{
System.out.println("bytes_read=" + bytes_read);
//this fails with the following error
//org.postgresql.util.PSQLException - FastPath call
returned ERROR: lo_write: invalid large obj descriptor (0)
lo.write(buff, 0, bytes_read);
}

//clean up
fis.close();
System.out.println("done");
}
catch (Exception e)
{
System.out.println(e.getClass().getName() + " - " +
e.getMessage());
}
finally
{
if (lo != null) try{lo.close();}catch (SQLException e){}
if (conn != null) try{conn.close();}catch (SQLException e){}
}
}
}