Can not import Unity IAP

Whenever i try to import Unity IAP it shows the error:

Code (CSharp):

1. InvalidCastException: Specified cast is not valid.
2. System.Net.AutoWebProxyScriptEngine.InitializeRegistryGlobalProxy () (at <0079a30f96a047348857e1cecc6c638a>:0)
3. System.Net.AutoWebProxyScriptEngine.GetWebProxyData () (at <0079a30f96a047348857e1cecc6c638a>:0)
4. System.Net.WebProxy.UnsafeUpdateFromRegistry () (at <0079a30f96a047348857e1cecc6c638a>:0)
5. System.Net.WebProxy..ctor (System.Boolean enableAutoproxy) (at <0079a30f96a047348857e1cecc6c638a>:0)
6. System.Net.WebProxy.CreateDefaultProxy () (at <0079a30f96a047348857e1cecc6c638a>:0)
7. System.Net.Configuration.DefaultProxySectionInternal.GetSystemWebProxy () (at <0079a30f96a047348857e1cecc6c638a>:0)
8. System.Net.Configuration.DefaultProxySectionInternal.GetDefaultProxy_UsingOldMonoCode () (at <0079a30f96a047348857e1cecc6c638a>:0)
9. System.Net.Configuration.DefaultProxySectionInternal.GetSection () (at <0079a30f96a047348857e1cecc6c638a>:0)
10. System.Net.WebRequest.get_InternalDefaultWebProxy () (at <0079a30f96a047348857e1cecc6c638a>:0)
11. System.Net.HttpWebRequest..ctor (System.Uri uri) (at <0079a30f96a047348857e1cecc6c638a>:0)
12. (wrapper remoting-invoke-with-check) System.Net.HttpWebRequest..ctor(System.Uri)
13. System.Net.HttpRequestCreator.Create (System.Uri uri) (at <0079a30f96a047348857e1cecc6c638a>:0)
14. System.Net.WebRequest.Create (System.Uri requestUri, System.Boolean useUriBase) (at <0079a30f96a047348857e1cecc6c638a>:0)
15. System.Net.WebRequest.Create (System.Uri requestUri) (at <0079a30f96a047348857e1cecc6c638a>:0)
16. System.Net.WebClient.GetWebRequest (System.Uri address) (at <0079a30f96a047348857e1cecc6c638a>:0)
17. System.Net.WebClient.DownloadFileAsync (System.Uri address, System.String fileName, System.Object userToken) (at <0079a30f96a047348857e1cecc6c638a>:0)
18. Rethrow as WebException: An exception occurred during a WebClient request.
19. UnityEditor.EditorApplication:Internal_CallUpdateFunctions()
20.  

Tried restarting Editor, also the issue occurs in an empty project too.