I do not understand the exception

[Jaap Bril <Jaap(dot)Bril(at)inter(dot)nl(dot)net>]

I dont get the error here, 

the exception is
	SQLEXCEPTION:Storing thumbnail failed: 
	java.sql.SQLException: ERROR:  Bad int8 external representation
	"\377\330\377\340\  [.........]

The offending code is:
    private void storeThumbnail(ConnectionPool connectionPool, byte[] tn) {
        String thumbnail_insert = "INSERT INTO thumbnails VALUES (?)";
        try {
            dbConn = connectionPool.getConnection();
            dbConn.setAutoCommit(false);
            PreparedStatement ps = dbConn.prepareStatement(thumbnail_insert);
            ps.setBytes(1, tn);
            ps.executeUpdate();
            ps.close();
        }
        catch (SQLException e){
            System.out.println("SQLEX:Storing thumbnail failed: " + e );
        }
    }


       tn is a 
		ByteArrayOutputStream().toByteArray() filled with a
		JPEGCodec.createJPEGEncoder()


The table thumbnails has a bytea and a serial;

What can be wrong??

TIA
Jaap