新硬盘怎么装系统:求助：关于ASP.Net 编程

小弟在用asp.net编写登陆系统页面，在实现登陆验证的后台代码时遇到报错：

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object

Source Error: Line 43:id = (Int32)myCommand.ExecuteScalar();

请问各位大虾们这是什么原因？？如何除错？？

其具体代码如下：
public int loginxp (String username,String password）
{
SqlConnection myConnection = new SqlConnection(ConfigurationSettings.AppSettings["SQLConnectionString"]);
SqlCommand myCommand = new SqlCommand("Pr_userlogin", myConnection);

myCommand.CommandType = CommandType.StoredProcedure;

SqlParameter parameterUsername = new SqlParameter("@username", SqlDbType.VarChar);
parameterUsername.Direction = ParameterDirection.Input;
parameterUsername.Value = username;
myCommand.Parameters.Add(parameterUsername);
SqlParameter parameterPassword = new SqlParameter("@password", SqlDbType.VarChar);
parameterPassword.Direction = ParameterDirection.Input;
parameterPassword.Value = password;
myCommand.Parameters.Add(parameterPassword);

Int32 id=0;
myConnection.Open();
id = (Int32)myCommand.ExecuteScalar();
return (int)id;
}