C#连接数据库代码问题?报错

private void btnLogin_Click(object sender, EventArgs e)
        {
            userName = txtUserName.Text;
            userPwd = txtUserPwd.Text;
            MySqlConnection conn = new MySqlConnection("server=localhost;database=person;uid=root;pwd=gjy3523347mysql");
            conn.Open();
            string sql2 = "select count(*) from per where name='" + userName + "' and pwd='" + userPwd + "'";
            if ((int)(new MySqlCommand(sql2,conn).ExecuteScalar()) > 0)
            {
                MessageBox.Show("登录成功!");
            }
            else {
                MessageBox.Show("登录失败!");
            }
}

为什么报错：System.InvalidCastException: 指定的转换无效。