diff --git a/lib/http/cookie_jar.rb b/lib/http/cookie_jar.rb
index 0889c07..0bacf68 100644
--- a/lib/http/cookie_jar.rb
+++ b/lib/http/cookie_jar.rb
@@ -9,8 +9,6 @@ end
 class HTTP::CookieJar
   include Enumerable
 
-  # add_cookie wants something resembling a URI.
-
   attr_reader :jar
 
   def initialize
@@ -23,6 +21,9 @@ class HTTP::CookieJar
 
   # Add a +cookie+ to the jar and return self.
   def add(cookie)
+    if cookie.domain.nil? || cookie.path.nil?
+      raise ArgumentError, "a cookie with unknown domain or path cannot be added"
+    end
     normal_domain = cookie.domain_name.hostname
 
     ((@jar[normal_domain] ||= {})[cookie.path] ||= {})[cookie.name] = cookie
diff --git a/test/test_http_cookie_jar.rb b/test/test_http_cookie_jar.rb
index aba2910..3a0a818 100644
--- a/test/test_http_cookie_jar.rb
+++ b/test/test_http_cookie_jar.rb
@@ -171,6 +171,18 @@ class TestHTTPCookieJar < Test::Unit::TestCase
     assert_equal(1, @jar.cookies(url).length)
   end
 
+  def test_add_rejects_cookies_with_unknown_domain_or_path
+    cookie = HTTP::Cookie.new(cookie_values.reject { |k,v| [:origin, :domain].include?(k) })
+    assert_raises(ArgumentError) {
+      @jar.add(cookie)
+    }
+
+    cookie = HTTP::Cookie.new(cookie_values.reject { |k,v| [:origin, :path].include?(k) })
+    assert_raises(ArgumentError) {
+      @jar.add(cookie)
+    }
+  end
+
   def test_add_does_not_reject_cookies_from_a_nested_subdomain
     url = URI 'http://y.x.foo.com'