How do I convert an ISO8601 TimeSpan to a C# TimeSpan?

I am getting a System.FormatException when I try to do the following (as an example):

TimeSpan ts = XmlConvert.ToTimeSpan("P72H");

I've investigated the ISO8601 Standard and it seems to be correct, but I cannot get it to parse hours without throwing an exception, no matter what I do.

Thanks!

EDIT:

Exception detail:

System.FormatException was unhandled by user code
HResult=-2146233033
Message=The string 'P72H' is not a valid TimeSpan value.
Source=System.Xml