added test for client SSL authentication
diff --git a/src/csharp/Grpc.IntegrationTesting/TestCredentials.cs b/src/csharp/Grpc.IntegrationTesting/TestCredentials.cs
index 401c50b..54d8587 100644
--- a/src/csharp/Grpc.IntegrationTesting/TestCredentials.cs
+++ b/src/csharp/Grpc.IntegrationTesting/TestCredentials.cs
@@ -78,7 +78,7 @@
             var keyCertPair = new KeyCertificatePair(
                 File.ReadAllText(ServerCertChainPath),
                 File.ReadAllText(ServerPrivateKeyPath));
-            return new SslServerCredentials(ImmutableList.Create(keyCertPair));
+            return new SslServerCredentials(new[] { keyCertPair });
         }
     }
 }