Ошибка при десериализации объекта - C# (183995)

<?xml version="1.0"?>
<Command xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <First>First</First>
  <Second>Second</Second>
  <Thrid>Thrid</Thrid>
  <Four>Four</Four>
</Command>

public class Command
        {
            [XmlElement("First")]
            public string FirstCommand;
            [XmlElement("Second")]
            public string SecondCommand;
            [XmlElement("Thrid")]
            public string ThridCommand;
            [XmlElement("Four")]
            public string FourCommand;
 
            public Command(string a, string b, string c, string d)
            {
                FirstCommand = a;
                SecondCommand = b;
                ThridCommand = c;
                Commandd = d;
            }
            public Command() { }
        }

            using (var stream = new MemoryStream())
            {
                var command = new Command("First","Second","Thrid","Four");
                var ser = new XmlSerializer(command.GetType());
                ser.Serialize(stream, command);
                ser.Deserialize(stream); // Здесь ошибка.                       
            }