diff --git a/src/format-helpers.js b/src/format-helpers.js
index 25581ab9..adcabddc 100644
--- a/src/format-helpers.js
+++ b/src/format-helpers.js
@@ -384,9 +384,11 @@ function amdGetCJSDeps(source, requireIndex) {
 function wrapEsModuleExecute (execute) {
   return function (require, exports, module) {
     execute(require, exports, module);
-    Object.defineProperty(module.exports, '__esModule', {
-      value: true
-    });
+    exports = module.exports;
+    if ((typeof exports === 'object' || typeof exports === 'function') && !('__esModule' in exports))
+      Object.defineProperty(module.exports, '__esModule', {
+        value: true
+      });
   };
 }
 
diff --git a/src/instantiate.js b/src/instantiate.js
index d18f19f4..565b59ad 100644
--- a/src/instantiate.js
+++ b/src/instantiate.js
@@ -12,7 +12,8 @@ if (typeof require !== 'undefined' && typeof process !== 'undefined' && !process
   nodeRequire = require;
 
 function setMetaEsModule (metadata, moduleValue) {
-  if (metadata.load.esModule && !('__esModule' in moduleValue))
+  if (metadata.load.esModule && (typeof moduleValue === 'object' || typeof moduleValue === 'function') &&
+      !('__esModule' in moduleValue))
     Object.defineProperty(moduleValue, '__esModule', {
       value: true
     });