don't promise MemoryStream, in case we can't do that in the future; Stream is fine

src/StackExchange.Redis/CompletionManager.cs

@@ -62,7 +62,8 @@ private static void SharedCompleteSyncOrAsync(ICompletable operation)
         }
         private void PerInstanceCompleteSyncOrAsync(ICompletable operation)
         {
-            if (operation.TryComplete(false))
+            if (operation == null) { }
+            else if (operation.TryComplete(false))
             {
                 multiplexer.Trace("Completed synchronously: " + operation, name);
                 Interlocked.Increment(ref completedSync);

src/StackExchange.Redis/ExtensionMethods.cs

@@ -170,7 +170,7 @@ private static void AuthenticateAsClientUsingDefaultProtocols(SslStream ssl, str
         /// </summary>
         /// <param name="bytes">The lease upon which to base the stream</param>
         /// <param name="ownsLease">If true, disposing the stream also disposes the lease</param>
-        public static MemoryStream AsStream(this Lease<byte> bytes, bool ownsLease = true)
+        public static Stream AsStream(this Lease<byte> bytes, bool ownsLease = true)
         {
             if (bytes == null) return null; // GIGO
             var segment = bytes.ArraySegment;