node中往mongo中插入数据的时候，明明插入成功了，但是会返回错误信息？

Question

exports.adduser = function(db) {
    return function(req, res) {
        // Get our form values. These rely on the "name" attributes
        var userName = req.body.username;
        var userEmail = req.body.useremail;
        // Set our collection
        var collection = db.get('users');
        // Submit to the DB
        collection.insert({
            "username": userName,
            "email": userEmail
        }, function(err, doc) {
            if (err) {
                res.send("There was a problem adding the information to the database.");
            } else {
                // If it worked, set the header so the address bar doesn't still say /adduser
                res.location("userlist");
                res.redirect("userlist");
            }
        });
    }
}

插入函数如上，我提交表单之后，查看数据库中是有数据的。但是会打印处if (err)的错误信息？这是什么原因造成的，我如果把res.location("userlist");res.redirect("userlist");移到if err里会报错，不会跳转