title: Convert units on rad button click error: System.InvalidCastException
body: <p>I am trying to use radio buttons on a calculator form in VB.net to on click convert from sq cm to sq in and vice versa but when i add the radio button click event to the code i keep getting this error :</p>
  
  <blockquote>
    <p>System.InvalidCastException: 'Conversion from string "" to type
    'Double' is not valid.'</p>
  </blockquote>
  
  <p>The click event subroutine looks like is this</p>
  
  <pre><code>Private Sub radSquareInch_CheckedChanged(sender As Object, e As EventArgs) Handles radSquareInch.CheckedChanged
      txtAnswer.Text = CDbl(txtAnswer.Text) * 0.155
  End Sub
  </code></pre>