diff --git a/Implab/Promise.cs b/Implab/Promise.cs
--- a/Implab/Promise.cs
+++ b/Implab/Promise.cs
@@ -178,7 +178,7 @@ namespace Implab {
 
         public Promise(IPromise parent) {
             if (parent != null)
-                AddMappers<T>(
+                AddHandler(
                     null,
                     null,
                     () => {
@@ -335,7 +335,7 @@ namespace Implab {
             var medium = new Promise<T>(this);
 
             AddMappers(
-                null,
+                x => x,
                 e => {
                     error(e);
                     return default(T);
@@ -362,7 +362,7 @@ namespace Implab {
 
             var medium = new Promise<T>(this);
 
-            AddMappers(null, handler, null, medium, true);
+            AddMappers(x => x, handler, null, medium, true);
 
             return medium;
         }