title: GlassMapper - GetItem<T>() method is failing to get item as specific type
body: <p>I'm trying to get item using GlassMapper as follow:</p>
  
  <pre><code>var myCustomObj=SitecoreContext.GetItem&lt;IMy_Custom_Type&gt;(itemId);
  </code></pre>
  
  <p>But <code>myCustomObj</code> is always null.</p>
  
  <p>When I try to get the item as <code>IGlass_Base</code> object, it works:</p>
  
  <pre><code>var myCustomObj=SitecoreContext.GetItem&lt;IGlass_Base&gt;(itemId);
  </code></pre>
  
  <p>Edited:
  Here is how the IMy_Custom_Type looks:</p>
  
  <pre><code>    /// &lt;summary&gt;
      /// IMy_Custom_Type Interface
      /// &lt;para&gt;&lt;/para&gt;
      /// &lt;para&gt;Path: /sitecore/templates/User Defined/###/Pages/My Custom Type&lt;/para&gt;    
      /// &lt;para&gt;ID: dfacd744-0cf8-4917-922c-4baeb07dfe35&lt;/para&gt;   
      /// &lt;/summary&gt;
      [SitecoreType(TemplateId=IMy_Custom_TypeConstants.TemplateIdString, AutoMap = true )] //, Cachable = true
      public partial interface IMy_Custom_Type : IGlassBase , global::SC.Global.Models.TemplateModels.Base_Templates.IMetadata, global::SC.Global.Models.TemplateModels.Base_Templates.IBase_Background_Image
      {
          /// &lt;summary&gt;
          /// The My Custom field.
          /// &lt;para&gt;&lt;/para&gt;
          /// &lt;para&gt;Field Type: Multilist&lt;/para&gt;      
          /// &lt;para&gt;Field ID: 43388fa5-a02a-425d-ae31-d94a12860748&lt;/para&gt;
          /// &lt;para&gt;Custom Data: &lt;/para&gt;
          /// &lt;/summary&gt;
          [SitecoreField(IMy_Custom_TypeConstants.MyCustomFieldName)]
          IEnumerable&lt;Guid&gt; My_Custom_Field  {get; set;}
          :
          :
          // Few more fields          
  
      }
  </code></pre>