List out of bounds (-1)

[henry64]

When the GSM have been running for some time I get an exception List index out of bounds (-1) in this procedure.

procedure TnrGsm.UpdateSignalInfo;
begin
 if CmdSendAndWaitByDriver(CID_GSM_GET_STATUS_SIGNAL) and (not ReplyLast.IsError)
   then DoUpdateSignalInfo(String(ReplyLast.sDataUseful),Driver.Command[ReplyLast.idDriverCmd]);
end;

The application polls for signalquality every 30 sec. but does nothing else - the connection stays open.

I have captured the exception
[05-01-2010 19:16:18] nrGsm: CmdSendAndWaitByDriver Error: List index out of bounds (-1), cmdid 209, j: -1