diff --git a/gulpfile.js b/gulpfile.js
index ae3cdee31..cb385f043 100644
--- a/gulpfile.js
+++ b/gulpfile.js
@@ -101,7 +101,7 @@ const paths = {
 
 
 const tsBaseArgs = {
-  target: "es5",
+  target: "es6",
   jsx: "react",
   reactNamespace: "React",
   experimentalDecorators: true,
diff --git a/src/checkable.ts b/src/checkable.ts
index 0e437839e..8e942d7cc 100644
--- a/src/checkable.ts
+++ b/src/checkable.ts
@@ -216,7 +216,8 @@ export namespace Checkable {
           type: target,
         }, ["(root)"]);
         if (opts.validate) {
-          if (target.validate !== "function") {
+          if (typeof target.validate !== "function") {
+            console.error("target", target);
             throw Error("invalid Checkable annotion: validate method required");
           }
           // May throw exception
diff --git a/tsconfig.json b/tsconfig.json
index ee2f58640..042ba3365 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "target": "es5",
+    "target": "es6",
     "jsx": "react",
     "reactNamespace": "React",
     "experimentalDecorators": true,