diff --git a/RestSharp/Http.Async.cs b/RestSharp/Http.Async.cs
index 50999f349..9d1c5f867 100644
--- a/RestSharp/Http.Async.cs
+++ b/RestSharp/Http.Async.cs
@@ -379,8 +379,10 @@ private HttpWebRequest ConfigureAsyncWebRequest(string method, Uri url)
 #if !PocketPC
 			webRequest.UseDefaultCredentials = UseDefaultCredentials;
 #endif
-			webRequest.PreAuthenticate = PreAuthenticate;
 
+#if !WINDOWS_PHONE && !SILVERLIGHT
+            webRequest.PreAuthenticate = PreAuthenticate;
+#endif
 			AppendHeaders(webRequest);
 			AppendCookies(webRequest);