diff --git a/HISTORY.md b/HISTORY.md
index 4bbaf17..1a7aed7 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -1,6 +1,7 @@
 unreleased
 ==========
 
+  * Fix `JSONCookie` to return `undefined` for non-string arguments
   * deps: cookie@0.1.5
 
 1.3.5 / 2015-05-19
diff --git a/index.js b/index.js
index 616b113..f511751 100644
--- a/index.js
+++ b/index.js
@@ -66,17 +66,19 @@ function cookieParser(secret, options) {
  * Parse JSON cookie string.
  *
  * @param {String} str
- * @return {Object} Parsed object or null if not json cookie
+ * @return {Object} Parsed object or undefined if not json cookie
  * @public
  */
 
 function JSONCookie(str) {
-  if (!str || str.substr(0, 2) !== 'j:') return;
+  if (typeof str !== 'string' || str.substr(0, 2) !== 'j:') {
+    return undefined;
+  }
 
   try {
     return JSON.parse(str.slice(2));
   } catch (err) {
-    // no op
+    return undefined;
   }
 }
 
diff --git a/test/cookieParser.js b/test/cookieParser.js
index 6001a4b..027d0bc 100644
--- a/test/cookieParser.js
+++ b/test/cookieParser.js
@@ -117,6 +117,16 @@ describe('cookieParser()', function(){
 })
 
 describe('cookieParser.JSONCookie(str)', function () {
+  it('should return undefined for non-string arguments', function () {
+    assert.strictEqual(cookieParser.JSONCookie(), undefined)
+    assert.strictEqual(cookieParser.JSONCookie(undefined), undefined)
+    assert.strictEqual(cookieParser.JSONCookie(null), undefined)
+    assert.strictEqual(cookieParser.JSONCookie(42), undefined)
+    assert.strictEqual(cookieParser.JSONCookie({}), undefined)
+    assert.strictEqual(cookieParser.JSONCookie([]), undefined)
+    assert.strictEqual(cookieParser.JSONCookie(function(){}), undefined)
+  })
+
   it('should return undefined for non-JSON cookie string', function () {
     assert.strictEqual(cookieParser.JSONCookie(''), undefined)
     assert.strictEqual(cookieParser.JSONCookie('foo'), undefined)