diff --git a/index.js b/index.js
index 493090e..46c2d9e 100644
--- a/index.js
+++ b/index.js
@@ -4,15 +4,11 @@ const mapAgeCleaner = require('map-age-cleaner');
 
 const cacheStore = new WeakMap();
 
-const mem = (fn, options = {}) => {
-	// Automatically use WeakMap unless the user provided their own cache
-	const weakCache = options.cache || new WeakMap();
-	const {
-		cacheKey = ([firstArgument]) => firstArgument,
-		cache = new Map(),
-		maxAge
-	} = options;
-
+const mem = (fn, {
+	cacheKey = ([firstArgument]) => firstArgument,
+	cache = new Map(),
+	maxAge
+} = {}) => {
 	if (typeof maxAge === 'number') {
 		mapAgeCleaner(cache);
 	}
@@ -20,18 +16,13 @@ const mem = (fn, options = {}) => {
 	const memoized = function (...arguments_) {
 		const key = cacheKey(arguments_);
 
-		// Prefer WeakMap if the key allows it
-		const bestCache = key && (typeof key === 'object' || typeof key === 'function') ?
-			weakCache :
-			cache;
-
-		if (bestCache.has(key)) {
-			return bestCache.get(key).data;
+		if (cache.has(key)) {
+			return cache.get(key).data;
 		}
 
 		const cacheItem = fn.apply(this, arguments_);
 
-		bestCache.set(key, {
+		cache.set(key, {
 			data: cacheItem,
 			maxAge: maxAge ? Date.now() + maxAge : Infinity
 		});
diff --git a/readme.md b/readme.md
index eb1b597..824ec47 100644
--- a/readme.md
+++ b/readme.md
@@ -190,9 +190,9 @@ Refer to the [caching strategies](#caching-strategy) section for more informatio
 ##### cache
 
 Type: `object`\
-Default: `new Map()`, but it also intelligently uses `new WeakMap()` whenevever possible
+Default: `new Map()`
 
-Use a different cache storage. Must implement the following methods: `.has(key)`, `.get(key)`, `.set(key, value)`, `.delete(key)`, and optionally `.clear()`. You could for example use [`quick-lru`](https://github.com/sindresorhus/quick-lru) for a LRU cache.
+Use a different cache storage. Must implement the following methods: `.has(key)`, `.get(key)`, `.set(key, value)`, `.delete(key)`, and optionally `.clear()`. You could for example use a `WeakMap` instead or [`quick-lru`](https://github.com/sindresorhus/quick-lru) for a LRU cache.
 
 Refer to the [caching strategies](#caching-strategy) section for more information.