Beep exception about message number

I am receiving this exception from Channel.postFrame, line 756:

 

org.beepcore.beep.core.Session.terminate: Incorrect message number: was 6;
expecting 5.

 

Is it a bug?

 

 

code:

                if (frame.getMsgno() != mstatus.getMsgno()) {

                    throw new BEEPException("Incorrect message number: was "

                                            + frame.getMsgno()

                                            + "; expecting "

                                            + mstatus.getMsgno());

                }

Serge