diff --git a/src/DotNetOpenAuth.OpenId/OpenId/XriDiscoveryProxyService.cs b/src/DotNetOpenAuth.OpenId/OpenId/XriDiscoveryProxyService.cs
index c8c126ed6c..bea8752dde 100644
--- a/src/DotNetOpenAuth.OpenId/OpenId/XriDiscoveryProxyService.cs
+++ b/src/DotNetOpenAuth.OpenId/OpenId/XriDiscoveryProxyService.cs
@@ -92,7 +92,9 @@ public class XriDiscoveryProxyService : IIdentifierDiscoveryService, IRequireHos
 
 			XrdsDocument doc;
 			using (var xrdsResponse = await Yadis.RequestAsync(GetXrdsUrl(identifier), identifier.IsDiscoverySecureEndToEnd, hostFactories, cancellationToken)) {
+				xrdsResponse.EnsureSuccessStatusCode();
 				var readerSettings = MessagingUtilities.CreateUntrustedXmlReaderSettings();
+				ErrorUtilities.VerifyProtocol(xrdsResponse.Content != null, "XRDS request \"{0}\" returned no response.", GetXrdsUrl(identifier));
 				await xrdsResponse.Content.LoadIntoBufferAsync();
 				using (var xrdsStream = await xrdsResponse.Content.ReadAsStreamAsync()) {
 					doc = new XrdsDocument(XmlReader.Create(xrdsStream, readerSettings));