diff --git a/lib/http/cookie.rb b/lib/http/cookie.rb
index 6aeffed..f44f59e 100644
--- a/lib/http/cookie.rb
+++ b/lib/http/cookie.rb
@@ -14,7 +14,14 @@ if RUBY_VERSION < "1.9.3"
     URI(URI(''))
   rescue
     def URI(url) # :nodoc:
-      url.is_a?(URI) ? url : URI.parse(url)
+      case url
+      when URI
+        url
+      when String
+        URI.parse(url)
+      else
+        raise ArgumentError, 'bad argument (expected URI object or URI string)'
+      end
     end
   end
 end