diff --git a/Implab/Promise.cs b/Implab/Promise.cs
--- a/Implab/Promise.cs
+++ b/Implab/Promise.cs
@@ -49,7 +49,7 @@ namespace Implab {
     /// </remarks>
     public class Promise<T> : IPromise<T> {
 
-        struct HandlerDescriptor {
+        protected struct HandlerDescriptor {
             public ResultHandler<T> resultHandler;
             public ErrorHandler errorHandler;
             public Action cancellHandler;