InvalidCastException:Specified cast is not valid

My GPS application can detect not only a Bluetooth GPS receiver but also a GPS/GSM Micro Tracker(it was connected to my application via TCP). I have to send SMS command and it reply back the GPS data(NMEA data - $GPRMC). The problem is that the application can display GPS data($GPGGA) from the Bluetooth GPS receiver but it can't display the data from the gsm machine. Whenever I send sms, I receive back the GPRMC data and I can show it in my textbox but I can't display it on my VE map. The following problem occurs.
http://img172.imagevenue.com/img.php?image=81285_problem2_122_505lo.JPG