diff --git a/Implab.Fx/PromiseHelpers.cs b/Implab.Fx/PromiseHelpers.cs
--- a/Implab.Fx/PromiseHelpers.cs
+++ b/Implab.Fx/PromiseHelpers.cs
@@ -25,7 +25,7 @@ namespace Implab.Fx
         ///         description => m_ctl.Text = description // now it's safe
         ///     )
         /// </example>
-        public static Promise<T> DispatchToControl<T>(this Promise<T> that, Control ctl)
+        public static Promise<T> DispatchToControl<T>(this IPromise<T> that, Control ctl)
         {
             if (that == null)
                 throw new ArgumentNullException("that");