TypeError: n is undefined

I'm getting this error on the page before the grid is even loaded. The offending code appears to be the middle line below.

var n = $('div.nav_blocker')[0];
-- n.style.height = $(jqNav).outerHeight()+"px"; --
n.style.width = ($(jqNav).outerWidth()+20)+"px"; 

I'm not declaring height on anything so I'm not sure what's causing it. Any help would be appreciated. I just sold Managment on using this grid instead of our "normal" grid which wasn't doing the job.

Thanks in Advance.