Close Serial Port After Data Received?

I have the following code that listens to a serial port and writes the incoming data to a listview control (the serial data are a delimited string taht arrives once per second that I parse into separate listview subitems). I would like to be able to close/disconnect the serial port once a specific value in the incoming string is detected. For example, in this case one of the columns of data in the incoming string represents depth, I would like to close the serial port when the depth falls below a given number. I have tried the code below, but my program crashes on the SerialPort.CLose method.

dim m_strOBS() as string
Private Sub SerialPort_OBS_DataReceived(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) _

[code].....