title: 'Argument 'Start' must be greater than zero. ' exception when using Mid function
body: <p>When I use Mid function </p>
  
  <pre><code>MsgBox(Mid(RXdata, RXdataLen - 3, 1))
  </code></pre>
  
  <p>in vb.net, I always get the following message </p>
  
  <pre><code>Additional information: Argument 'Start' must be greater than zero.
  </code></pre>
  
  <p>I think it is impossible since the RdataLen is 9, the 'Start' is definitely larger than 9.  RXdata is the data that I read from SerialPort.</p>
  
  <p>Any thought on how come it happens?</p>
  
  <p>Thanks</p>