title: WPF DataBinding: Nullable Int still gets a validation error?
body: <p>I have a textbox databound to a nullable int through code. If I erase the data from the textbox it gives me a validation error (red border around it).</p>
  
  <p>Here is my binding code:</p>
  
  <pre><code>ZipBinding = new Binding("Zip");
  ZipBinding.Source = Address;
  zipTextBox.SetBinding(TextBox.TextProperty, ZipBinding);
  
  public Int32? Zip { get { ... } set { ... } }
  </code></pre>
  
  <p>It's clearly marked as a Nullable so why does WPF wanna give me a validation issue when I clear the textbox?</p>