diff --git a/README.md b/README.md
index 3fd6d0bc..c06814e0 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,24 @@ If you put more stuff in it, then items will fall out.
 If you try to put an oversized thing in it, then it'll fall out right
 away.
 
+## Keys should always be Strings or Numbers
+
+Note: this module will print warnings to `console.error` if you use a
+key that is not a String or Number.  Because items are stored in an
+object, which coerces keys to a string, it won't go well for you if
+you try to use a key that is not a unique string, it'll cause surprise
+collisions.  For example:
+
+```JavaScript
+// Bad Example!  Dont' do this!
+var cache = LRU()
+var a = {}
+var b = {}
+cache.set(a, 'this is a')
+cache.set(b, 'this is b')
+console.log(cache.get(a)) // prints: 'this is b'
+```
+
 ## Options
 
 * `max` The maximum size of the cache, checked by applying the length
diff --git a/lib/lru-cache.js b/lib/lru-cache.js
index 32c2d2d9..2bbe653b 100644
--- a/lib/lru-cache.js
+++ b/lib/lru-cache.js
@@ -13,6 +13,14 @@ function hOP (obj, key) {
 
 function naiveLength () { return 1 }
 
+var didTypeWarning = false
+function typeCheckKey(key) {
+  if (!didTypeWarning && typeof key !== 'string' && typeof key !== 'number') {
+    didTypeWarning = true
+    console.error(new TypeError("LRU: key must be a string or number. Almost certainly a bug! " + typeof key).stack)
+  }
+}
+
 function LRUCache (options) {
   if (!(this instanceof LRUCache))
     return new LRUCache(options)
@@ -163,6 +171,8 @@ LRUCache.prototype.dumpLru = function () {
 
 LRUCache.prototype.set = function (key, value, maxAge) {
   maxAge = maxAge || this._maxAge
+  typeCheckKey(key)
+
   var now = maxAge ? Date.now() : 0
   var len = this._lengthCalculator(value)
 
@@ -207,6 +217,7 @@ LRUCache.prototype.set = function (key, value, maxAge) {
 }
 
 LRUCache.prototype.has = function (key) {
+  typeCheckKey(key)
   if (!hOP(this._cache, key)) return false
   var hit = this._cache[key]
   if (isStale(this, hit)) {
@@ -216,10 +227,12 @@ LRUCache.prototype.has = function (key) {
 }
 
 LRUCache.prototype.get = function (key) {
+  typeCheckKey(key)
   return get(this, key, true)
 }
 
 LRUCache.prototype.peek = function (key) {
+  typeCheckKey(key)
   return get(this, key, false)
 }
 
@@ -230,6 +243,7 @@ LRUCache.prototype.pop = function () {
 }
 
 LRUCache.prototype.del = function (key) {
+  typeCheckKey(key)
   del(this, this._cache[key])
 }
 
@@ -241,6 +255,7 @@ LRUCache.prototype.load = function (arr) {
   //A previous serialized cache has the most recent items first
   for (var l = arr.length - 1; l >= 0; l-- ) {
     var hit = arr[l]
+    typeCheckKey(hit.k)
     var expiresAt = hit.e || 0
     if (expiresAt === 0) {
       //the item was created without expiration in a non aged cache
@@ -254,6 +269,7 @@ LRUCache.prototype.load = function (arr) {
 }
 
 function get (self, key, doUse) {
+  typeCheckKey(key)
   var hit = self._cache[key]
   if (hit) {
     if (isStale(self, hit)) {