异常消息=“对象引用未设置为对象的实例。”

Question

我正在尝试使用.NET执行UPDATE存储过程。 这是C＃部分：

     public int spUpdate_Agent(int agentNum, string firstName, string lastNAme, string city, string phone
   , string eMail)
{
    string cStr = WebConfigurationManager.ConnectionStrings["bgroup33_prodConnectionString"].ConnectionString;
    using (SqlConnection con = new SqlConnection(cStr))
    {
        SqlCommand cmd = new SqlCommand("spUpdate_Agent", con);
        cmd.CommandType = CommandType.StoredProcedure;

        SqlParameter paramAgentNum = new SqlParameter("@agentNum", agentNum);
        SqlParameter paramAgentFirst = new SqlParameter("@FirstName", firstName);
        SqlParameter paramAgentLast = new SqlParameter("@lasttName", lastNAme);
        SqlParameter paramAgentCity = new SqlParameter("@city", city);
        SqlParameter paramAgentPhone = new SqlParameter("@phone", phone);
        SqlParameter paramAgentEmail = new SqlParameter("@eMail", eMail);

        cmd.Parameters.Add(paramAgentNum);
        cmd.Parameters.Add(paramAgentFirst);
        cmd.Parameters.Add(paramAgentLast);
        cmd.Parameters.Add(paramAgentCity);
        cmd.Parameters.Add(paramAgentPhone);
        cmd.Parameters.Add(paramAgentEmail);

        try
        {
            con.Open();
            int returenCode = (int)cmd.ExecuteScalar();
            return returenCode;
        }
        catch (Exception ex)
        {
            return -1;

        }
        finally
        {
            con.Close();
        }
    }
}

这就是存储过程：

alter procedure [dbo].[spUpdate_Agent]
@agentNum int,
@firstName nvarchar(50),
@lasttName nvarchar(50),
@city nvarchar(50),
@phone nvarchar(50),
@eMail nvarchar(50)

    as
    Begin 

        update [dbo].[Agent]
        set first_name = @firstName,last_name = @lasttName, city = @city,phone = @phone,eMail = @eMail
        where agentNum = @agentNum
        return 1
    end

所以我得到如上所述的这个错误，所有参数都是正确的类型。 那我的问题是什么？