diff --git a/index.js b/index.js
index 0f289f8..01e0b3c 100644
--- a/index.js
+++ b/index.js
@@ -35,7 +35,31 @@ try {
   nodeInspect = false
 }
 
-const constructorMap = new WeakMap()
+function FakeMap() {
+  // eslint-disable-next-line prefer-template
+  this.key = 'chai/loupe__' + Math.random() + Date.now()
+}
+FakeMap.prototype = {
+  // eslint-disable-next-line object-shorthand
+  get: function get(key) {
+    return key[this.key]
+  },
+  // eslint-disable-next-line object-shorthand
+  has: function has(key) {
+    return this.key in key
+  },
+  // eslint-disable-next-line object-shorthand
+  set: function set(key, value) {
+    if (Object.isExtensible(key)) {
+      Object.defineProperty(key, this.key, {
+        // eslint-disable-next-line object-shorthand
+        value: value,
+        configurable: true,
+      })
+    }
+  },
+}
+const constructorMap = new (typeof WeakMap === 'function' ? WeakMap : FakeMap)()
 const stringTagMap = {}
 const baseTypesMap = {
   undefined: (value, options) => options.stylize('undefined', 'undefined'),
@@ -178,7 +202,7 @@ export function registerConstructor(constructor, inspector) {
   if (constructorMap.has(constructor)) {
     return false
   }
-  constructorMap.add(constructor, inspector)
+  constructorMap.set(constructor, inspector)
   return true
 }