title: WPF Textbox accept INT but not NULLABLE INT?
body: <p>Model</p>
  
  <pre><code>public int? SizeLength { get; set; }
  </code></pre>
  
  <p>XAML</p>
  
  <pre><code>&lt;TextBox Text="{Binding [someViewModel].SizeLength, Mode=TwoWay}"&gt;&lt;/TextBox&gt;
  </code></pre>
  
  <p>Once user try to <kbd>backspace</kbd> or <kbd>delete</kbd> the value in this <code>textbox</code>, a message <code>Value '' cannot be converted</code>. </p>
  
  <p>May I know what's wrong with it?</p>