about cache class increment and decrement bugs?

when i use Phalcon\Cache\Backend\Redis the increment and decrement method. if i wrote.

    $this->redis->increment('aaa',1);
    $result = $this->redis->get('aaa');
     echo $result;

the $result is normal.

but, i wrote that.

    $this->redis->increment('aaa',1);
    var_dump($this->redis->get('aaa'));

the return value ,Every time increment values is 2. why?