npgsql :选择null数据引发异常，错误为"Column is Null“

using Npgsql;
using System;

class Program
{
    static void Main()
    {
        string connectionString = "Host=myserver;Username=mylogin;Password=mypassword;Database=mydatabase";

        using (NpgsqlConnection conn = new NpgsqlConnection(connectionString))
        {
            conn.Open();

            string sql = "SELECT Email FROM Users WHERE Id = @UserId";
            using (NpgsqlCommand cmd = new NpgsqlCommand(sql, conn))
            {
                cmd.Parameters.AddWithValue("UserId", 1);

                using (NpgsqlDataReader reader = cmd.ExecuteReader())
                {
                    if (reader.Read())
                    {
                        // 使用可空类型来接收可能为 null 的值
                        string? email = reader.GetString(0);

                        // 进行 null 检查
                        if (email != null)
                        {
                            Console.WriteLine($"Email: {email}");
                        }
                        else
                        {
                            Console.WriteLine("Email is not available.");
                        }
                    }
                }
            }
        }
    }
}