Memcached::get() expects parameter 2 to be a valid callback

With php-fpm 5.5.3 and memcached 1.4.14 I'm seeing the following warning several times per request on WordPress 3.8.1:

```
PHP message: PHP Warning:  Memcached::get() expects parameter 2 to be a valid callback, no array or string given in /usr/share/wordpress/wp-content/object-cache.php on line 1262
```