Scroller and serverside - length always '-1' and start always '0'

Hi! I use scroller with this options:

     searching: true,
     paging: false,
     "sDom": "t",
     "bFilter": false,
     "info": false,
     "sScrollY": 200,
    "oScroller": {
       "rowHeight": 20,
    },
    "ajax": {
       url: '/echo/json/',
       type: 'POST'
    },
      stateSave: true,
    "processing": true,
     "serverSide": true,
  }

table height is 200 so I suppose that POST dict on my server will contain key 'length' with value 200 / 20(row height) = 10 + 10 (buffered), but it is always '-1'. Is there some config error?

Here is a fiddle http://jsfiddle.net/HEDvf/2712/ (In Google chrome Dev tools Network -> XHR -> find 'json/' -> Headers tab -> See Form Data)