function send_request(url,str) {
    var http_request = false;
    if(window.XMLHttpRequest) {
        http_request = new XMLHttpRequest();
        if (http_request.overrideMimeType){
            http_request.overrideMimeType('text/xml');
        }
    }else if(window.ActiveXObject){
        try {http_request = new ActiveXObject("Msxml2.XMLHTTP");}catch (e){
            try {http_request = new ActiveXObject("Microsoft.XMLHTTP");} catch (e) {}
        }
    }
    if (!http_request) {
        window.alert("不能创建XMLHttpRequest对象实例.");
        return false;
    }
    http_request.open("GET",url, true);
    http_request.onreadystatechange=function(){
        if (http_request.readyState == 4) {
            if (http_request.status == 200) {
                document.getElementById(str).innerHTML= http_request.responseText;
            } else {
                alert("您所请求的页面有异常。");
            }
        }
    }
　    http_request.send(null);
}
运行这段代码时，提示说“您所请求的页面有异常”请问：如果要避免出现这个提示，这个函数所输入的参数有什么要求，或者说，用什么办法能避免出现这个提示