diff --git a/README.md b/README.md
index f7c12f8..8b0b3ac 100644
--- a/README.md
+++ b/README.md
@@ -61,7 +61,7 @@ If the signature cookie _does_ exist, the provided [Keygrip](https://www.npmjs.c
 
 This sets the given cookie in the response and returns the current context to allow chaining.
 
-If the _value_ is omitted, an outbound header with an expired date is used to delete the cookie.
+If the _value_ is omitted (or `null`), an outbound header with an expired date is used to delete the cookie. If _value_ is not omitted, it _must_ be given as string.
 
 If the _options_ object is provided, it will be used to generate the outbound cookie header as follows:
 
diff --git a/index.js b/index.js
index 9c468ae..d4f0d00 100644
--- a/index.js
+++ b/index.js
@@ -121,6 +121,10 @@ function Cookie(name, value, attrs) {
     throw new TypeError('argument name is invalid');
   }
 
+  if (value !== undefined && value !== null && typeof value !== 'string') {
+    throw new Error('argument value is not a string');
+  }
+
   if (value && !fieldContentRegExp.test(value)) {
     throw new TypeError('argument value is invalid');
   }
diff --git a/test/cookie.js b/test/cookie.js
index 347c2e8..e6d88c5 100644
--- a/test/cookie.js
+++ b/test/cookie.js
@@ -32,6 +32,18 @@ describe('new Cookie(name, value, [options])', function () {
     }, /option domain is invalid/)
   })
 
+  it('should throw on number value', function () {
+    assert.throws(function () {
+      new cookies.Cookie('foo', 0)
+    }, /argument value is not a string/)
+  })
+
+  it('should throw on boolean value', function () {
+    assert.throws(function () {
+      new cookies.Cookie('foo', false)
+    }, /argument value is not a string/)
+  })
+
   describe('options', function () {
     describe('maxage', function () {
       it('should set the .maxAge property', function () {