Proxies return private properties in __sleep, which is not supported by PHP.

class SomeGeneratedProxyName extends RealName implements \Serializable
{
    public function unserialize($data)
    {
        $reflectionClass = new ReflectionClass($this);
        foreach ($reflectionClass->getProperties(ReflectionProperty::IS_PRIVATE) as $privateProp) {
            $privateProp->setAccessible(true);
            $privateProp->setValue($this, $data[$privateProp->getName()]);
        }
        // ... other props ...
    }

    public function serialize()
    {
        $data = array();
        $reflectionClass = new ReflectionClass($this);
        foreach ($reflectionClass->getProperties(ReflectionProperty::IS_PRIVATE) as $privateProp) {
            $privateProp->setAccessible(true);
            $data[$privateProp->getName()] = $privateProp->getValue($this);
        }
        // ... other props ...
        return $data;
    }
}

class MyEntity implements Serializable
{
    public function serialize()
    {
        // [...]
    }

    public function unserialize($serialized)
    {
        // [...]
    }

    public function __sleep()
    {
        // [...]
    }

    public function __wakeup()
    {
        // [...]
    }
}

chr(0) . 'Foo' . chr(0) . 'bar'

array_keys((array) $object);