Init a list to empty

In [bourbon](http://thoughtbot.com/bourbon/) and specificaly in [html5-input-types](https://github.com/thoughtbot/bourbon/blob/master/app/assets/stylesheets/addons/_html5-input-types.scss) there is an empty list which is then populated in an @each loop.

Phpsass crashes on the first append call in this loop :
    Call to undefined method SassBoolean::append() in .../phpsass/script/SassScriptFunctions.php on line 901

I managed to get this working making an ugly special case to the append method, but this does not seem a good option to me. The problem seems to be from the lexer, if I understand. But I don't know how to solve this bug in a good way ...