Collection of tr objects (rows) in a table with IE8

var oTable = document.getElementById("tbl");
    for (var i = 0; i < oTable.rows.length; i++)
    {
        oTable.rows[i].style.fontWeight = "bold";
    }

Code (markup):

tbl is ID of <table>
with IE run error : rows is null

how can i use Collection of row objects (rows) in a table with IE8 ?
with IE7 , 6 no errors.