diff --git a/index.js b/index.js
index dd6d479..1c934da 100644
--- a/index.js
+++ b/index.js
@@ -109,7 +109,11 @@ function JSONCookies (obj) {
     key = cookies[i]
     val = JSONCookie(obj[key])
 
-    if (val) {
+    // Boolean false is reserved: signedCookies() uses it to mark a cookie whose
+    // signature did not verify, so writing a parsed "j:false" here would make an
+    // authentic value indistinguishable from a tampered one (#168). Left as the
+    // raw string until that representation is settled.
+    if (val !== undefined && val !== false) {
       obj[key] = val
     }
   }
diff --git a/test/cookieParser.js b/test/cookieParser.js
index 6031823..6226f59 100644
--- a/test/cookieParser.js
+++ b/test/cookieParser.js
@@ -81,6 +81,23 @@ describe('cookieParser()', function () {
         .expect(200, '{"foo":"foobarbaz"}', done)
     })
 
+    it('should keep an authentic JSON false distinct from a tampered cookie', function (done) {
+      var signed = signature.sign('j:false', 'keyboard cat')
+      request(createServer('keyboard cat'))
+        .get('/signed')
+        .set('Cookie', 'flag=s:' + signed)
+        .expect(200, '{"flag":"j:false"}', done)
+    })
+
+    it('should report a tampered cookie as false', function (done) {
+      var signed = signature.sign('j:false', 'keyboard cat')
+      var tampered = signed.slice(0, -1) + (signed.slice(-1) === 'a' ? 'b' : 'a')
+      request(createServer('keyboard cat'))
+        .get('/signed')
+        .set('Cookie', 'flag=s:' + tampered)
+        .expect(200, '{"flag":false}', done)
+    })
+
     it('should remove the signed value from req.cookies', function (done) {
       request(createServer('keyboard cat'))
         .get('/')
@@ -162,6 +179,24 @@ describe('cookieParser.JSONCookie(str)', function () {
   })
 })
 
+describe('cookieParser.JSONCookies(obj)', function () {
+  it('should parse falsy JSON cookie values', function () {
+    assert.deepEqual(cookieParser.JSONCookies({
+      z: 'j:0',
+      e: 'j:""',
+      n: 'j:null'
+    }), { z: 0, e: '', n: null })
+  })
+
+  it('should leave a JSON false as its raw string, so it stays distinct from the signature-failure marker', function () {
+    assert.deepEqual(cookieParser.JSONCookies({ f: 'j:false' }), { f: 'j:false' })
+  })
+
+  it('should leave non-JSON cookie values untouched', function () {
+    assert.deepEqual(cookieParser.JSONCookies({ a: 'bar', b: 'j:{foo:"bar"}' }), { a: 'bar', b: 'j:{foo:"bar"}' })
+  })
+})
+
 describe('cookieParser.signedCookie(str, secret)', function () {
   it('should return undefined for non-string arguments', function () {
     assert.strictEqual(cookieParser.signedCookie(undefined, 'keyboard cat'), undefined)