C#NET开发Winform使用MySQL数据库 无法将“System.Byte[]”的对象强制转换为类型“System.IConvertible”,该怎么处理

private const string connectString = "server=127.0.0.1;uid=root;pwd=root;database=dbname";
public static bool MysqlExcute(MySqlCommand cmd)
{
    MySqlConnection conn = new MySqlConnection(connectString);
    conn.Open();
    cmd.Connection = conn;
    int rtn = cmd.ExecuteNonQuery();
    conn.Close();
    return rtn > 0;
}

                // 读取图片内容
                FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
                Byte[] imgByte = new Byte[fs.Length];                
                fs.Read(imgByte, 0, imgByte.Length);

                // 插入图片
                MySqlCommand cmd = new MySqlCommand("INSERT INTO pic(pic_data, upload_username, upload_datetime) " +
                    "VALUES(?pic_name, ?pic_data, ?upload_username, ?upload_datetime)");
                cmd.Parameters.AddWithValue("?pic_name", pic_name);
                cmd.Parameters.AddWithValue("?pic_data", MySqlDbType.VarBinary).Value = imgByte;
                cmd.Parameters.AddWithValue("?upload_username", Session.username);
                cmd.Parameters.AddWithValue("?upload_datetime", DateTime.Now.ToString());
                if (MySqlHelper.MysqlExcute(cmd))
                {
                    // 上传成功
                    // ...
                }
                else
                {
                    // ...
                }

MySqlParameter param = new MySqlParameter();
param.MySqlDbType = MySqlDbType.VarBinary;
param.Value = imgByte;
cmd.Parameters.Add(param);