diff --git a/lib/6to5/polyfill.js b/lib/6to5/polyfill.js
index 4934fdb59b..2cf4d1784d 100644
--- a/lib/6to5/polyfill.js
+++ b/lib/6to5/polyfill.js
@@ -30,6 +30,8 @@ ensureProto(Map, Symbol.referenceGet, Map.prototype.get);
 ensureProto(Map, Symbol.referenceSet, Map.prototype.set);
 ensureProto(Map, Symbol.referenceDelete, Map.prototype.delete);
 
-ensureProto(WeakMap, Symbol.referenceGet, WeakMap.prototype.get);
-ensureProto(WeakMap, Symbol.referenceSet, WeakMap.prototype.set);
-ensureProto(WeakMap, Symbol.referenceDelete, WeakMap.prototype.delete);
+if (global.WeakMap) {
+  ensureProto(WeakMap, Symbol.referenceGet, WeakMap.prototype.get);
+  ensureProto(WeakMap, Symbol.referenceSet, WeakMap.prototype.set);
+  ensureProto(WeakMap, Symbol.referenceDelete, WeakMap.prototype.delete);
+}