diff --git a/index.js b/index.js
index 78e1e96..1e93bb3 100644
--- a/index.js
+++ b/index.js
@@ -2,19 +2,30 @@
 // much λ, much UX.
 module.exports = function λ(fn) {
   return function(e, ctx, cb) {
+    var succeed, fail;
+    // cb will be defined for node 4.3 on lambda
+    if (cb) {
+      succeed = function(result) {
+        cb(null, result)
+      }
+      fail = function(err) {
+        cb(err)
+      }
+    } else {
+      succeed = ctx.succeed, fail = ctx.fail;
+    }
+
     try {
       var v = fn(e, ctx, cb)
 
       if (v && typeof v.then == 'function') {
-        v.then(function (val) {
-            cb(null, val)
-        }).catch(cb)
+        v.then(succeed).catch(fail)
         return
       }
 
-      cb(null, v)
+      succeed(v)
     } catch (err) {
-      cb(err)
+      fail(err)
     }
   }
 }