I do not understand the exception

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