title: ASP.net MVC - Custom attribute error message with nullable properties
body: <p>I am having a property in my View Model which can accept integer and nullable values:</p>
  
  <pre><code>    [Display(Name = "Code Postal")]
      public int? CodePostal { get; set; }
  </code></pre>
  
  <p>When I type in string values, how can display another message than the default one:</p>
  
  <pre><code>The field Code Postal must be a number.
  </code></pre>
  
  <p>Thanks</p>