IE8 fires off error messgage

[eberdome]

hello there, I have some code that is firing off an error message in IE8 that states " 'undefined' is null or not an object' on line 22 char 16. Anyone know why

/**
 * @author Alexander Farkas
 * v. 1.02
 */
(function($) {
	$.extend($.fx.step,{
	    backgroundPosition: function(fx) {
            if (fx.state === 0 && typeof fx.end == 'string') {
                var start = $.curCSS(fx.elem,'backgroundPosition');
                start = toArray(start);
                fx.start = [start[0],start[2]];
                var end = toArray(fx.end);
                fx.end = [end[0],end[2]];
                fx.unit = [end[1],end[3]]
			}
            var nowPosX = [];
            nowPosX[0] = ((fx.end[0] - fx.start[0]) * fx.pos) + fx.start[0] + fx.unit[0];
            nowPosX[1] = ((fx.end[1] - fx.start[1]) * fx.pos) + fx.start[1] + fx.unit[1];
            fx.elem.style.backgroundPosition = nowPosX[0]+' '+nowPosX[1];

           function toArray(strg){
               strg = strg.replace(/left|top/g,'0px');
               strg = strg.replace(/right|bottom/g,'100%');
               strg = strg.replace(/([0-9\.]+)(\s|\)|$)/g,"$1px$2");
               var res = strg.match(/(-?[0-9\.]+)(px|\%|em|pt)\s(-?[0-9\.]+)(px|\%|em|pt)/);
               return [parseFloat(res[1],10),res[2],parseFloat(res[3],10),res[4]];
           }
        }
	});
})(jQuery);

Thanks in advance!

[AndrewGSW]

Shouldn't it be

               strg = strg.replace(/[left|top]/g,'0px');
               strg = strg.replace(/[right|bottom]/g,'100%');

Is 'fx.elem' correctly defined?