Can Recovery with XmlRead

I test the XmlWrite/XmlRead from MemoryStrem, my codes is:
\
DataSet ds = Will.DM.ExecuteDataset("Select * from Customer);
MemoryStream msOri = new MemoryStream();
ds.WriteXml(msOri);

DataSet ds2 = new DataSet();
ds2.ReadXml(msOri);

It compiled ok, but when it run fail at the last line: ds.WriteXml(msOri);
Ther error message is
System.Xml.XmlException: Root element is missing.

Why?