diff --git a/index.js b/index.js
index f095fb9..c9ca677 100644
--- a/index.js
+++ b/index.js
@@ -3,27 +3,27 @@
 var $Map = typeof Map === 'function' && Map.prototype ? Map : null;
 var $Set = typeof Set === 'function' && Set.prototype ? Set : null;
 
+var exported;
+
 if (!$Set) {
 	// eslint-disable-next-line no-unused-vars
-	module.exports = function isSet(x) {
+	exported = function isSet(x) {
 		// `Set` is not present in this environment.
 		return false;
 	};
-	return;
 }
 
 var $mapHas = $Map ? Map.prototype.has : null;
 var $setHas = $Set ? Set.prototype.has : null;
-if (!$setHas) {
+if (!exported && !$setHas) {
 	// eslint-disable-next-line no-unused-vars
-	module.exports = function isSet(x) {
+	exported = function isSet(x) {
 		// `Set` does not have a `has` method
 		return false;
 	};
-	return;
 }
 
-module.exports = function isSet(x) {
+module.exports = exported || function isSet(x) {
 	if (!x || typeof x !== 'object') {
 		return false;
 	}