Dns.GetHostEntry returns a nonexistent name

My C# code, string szHostName = Dns.GetHostEntry(e.SourceAddress).HostName; (e.SourceAddress is a returned arguement of tyrpe string from a mcast event), is returning a name that does not exist in my system.  I'm guessing it's not a C# issue, but a DNS issue.  I changed the IP of the machine that I wasn't getting the name for and that worked, but that doesn't really fix the problem.  I need to put the original IP back and figure out why I'm getting back a nonexistent name.

Thanks