javascript滚动特效错误

下面的代码提示demo未定义错误 怎么改
<div id="demo" style="overflow:hidden;width:200px;border:2px solid #e0e0e0;padding:2px;" onmouseover="stopscroll();" onmouseout="doscroll()">
<div id="demo1" style="white-space:nowrap;padding:0;">
<a href="#"><img src="0.jpg" height="30" width="100" border="0"></a>
<a href="#"><img src="0.jpg" height="30" width="100" border="0"></a>
</div>
</div>
<script type="text/javascript">
var t=demo.scrollWidth
demo1.innerHTML+=demo1.innerHTML
function doMarquee(){
  demo.scrollLeft=demo.scrollLeft<demo.scrollWidth-demo.offsetWidth?demo.scrollLeft+1:t-demo.offsetWidth;
}
function doscroll(){
  sc=setInterval(doMarquee,20);
}
function stopscroll(){
  clearInterval(sc);
}
doscroll();
</script>