title: JsonSchema.Validate() ObjectExpected: #/myField
body: <p>I'm trying to validate some json using the JSON.Net Schema support for validation. The JSON has a field called extra which is optional - sometimes it will be supplied with null. I'm trying the following code:</p>
  
  <pre><code>var json = "{ \"id\": 1, \"extra\": null }";
  
  var schema = new JsonSchema4();
  var extraProp = new JsonProperty {
      Type = JsonObjectType.Object,
      IsRequired = false
  };
  
  schema.Properties.Add( "extra", extraProp );
  
  schema.Validate(json);
  </code></pre>
  
  <p>And am receiving the following validation error:</p>
  
  <pre><code>ObjectExpected: #/extra
  </code></pre>
  
  <p>How can I set the schema up to tolerate the "extra" field being null sometimes?</p>