setting mootools fx.morph height to auto instead of a fix height -

i'm trying show nav using fx.morph code below:

 var fx = new fx.morph(_element, {       duration: 1000,transition:fx.transitions.sine.easeout                                  });  fx.start({           'height': [0, 100]            });  } 

it works well,the thing need set height 0 auto instead of 0 100.is there solution that?i try auto , 100% didn't work.

i've done myself.i add container , after calculat height of container , height.

    var _element = el.getelement('.child_first_nav_mobile');             if (_element) {                  var fx = new fx.morph(_element, { duration: 1000 });                 var autoheightdiv = _element;                 autoheightdiv.style.height='auto';                 var calcheight=autoheightdiv.offsetheight                 fx.start({                     'height': [0, calcheight]                      });