Liquid error: undefined method `join' for "\nputs \"Hello Jekyll!\"\n":String

[izuoshi]

Hello all:

I just start to use jekyll to blog, but whenever I use highlight
code, for example

{% highlight ruby %}
puts "Hello Jekyll!"
{% endhighlight %}

The result page just displays :

Liquid error: undefined method `join' for "\nputs \"Hello Jekyll!
\"\n":String

do you have any ideas? (I have set pygments: true in the _config.yml
file)

thanks