这段运算代码哪里错了？
<!DOCTYPE html>
<html>
<head>
<title> 事件</title>
<script type="text/javascript">
function count(){
var t1 = document.getElementById("txt1").value;
var t2 =document.getElementById("txt2").value;
var sel = document.getElementById("select").value;
var sum;
switch(sel){
case "+":sum = parseInt(t1)+parseInt(t2);break;
case "-":sum = parseInt(t1)-parseInt(t2);break;
case "*":sum = parseInt(t1)*parseInt(t2);break;
case "/":sum = parseInt(t1)/parseInt(t2);break;
}
document.getElementById("fruit").value = sum;
}
</script>
</head>
<body>
<input type='text' id='txt1' />
<select id='select'>
<option value='+'>+</option>
<option value="-">-</option>
<option value="*">*</option>
<option value="/">/</option>
</select>
<input type='text' id='txt2' />
<input type='button' value=' = ' /> <!--通过 = 按钮来调用创建的函数，得到结果-->
<input type='text' id='fruit' />
</body>
