jQuery Grid Plugin - jqGrid

function getCellIndex exit with error: TypeError c is undefined

jQuery v1.7

firefox 14.0.1

I get this error when click on cell:

Cell define:
function _formatterControl (cellvalue, options, rowObject) {
        var div = '<img class="checkbox" src="/blog/';
        if (!cellvalue) {
            div += checkboxoff;
        }else {
            div += checkboxon;
        }
        div += '" alt="" />';
        return div;

}

.....
{
                    name:'control',
                    align: 'center',
                    width: '18px',
                    sortable: false,
                    formatter:_formatterControl
},
..

the function: 

getCellIndex : function (cell) {
            var c = $(cell);
            if (c.is('tr')) {
                return -1;
            }
            c = (
                !c.is('td')
                &&
                !c.is('th') ? c.closest("td,th") : c
                )[0];
            console.log (c); --- here I get undefined
            if ($.browser.msie) {
                return $.inArray(c, c.parentNode.cells);
            }
            return c.cellIndex;
        }