adding custom properties to a user object

[nelis]

Anyone ever ran into the issue of adding custom properties to a user? Here's a code snippet i wrote:

DirectoryEntry newUser = deContainer.Children.Add("CN=" + username, "User");
newUser.Invoke("Put", new object[] { "sAMAccountName", "testuser" });
newUser.Invoke("Put", new object[] { "sn", "user1" });
newUser.Invoke("Put", new object[] { "testprop1", "will work" });   //custom property
newUser.CommitChanges();
newUser.Close();
 

This code makes the user fine if I remove the custom property. When I leave it in, and it tries to commit, I get the exception:
The specified directory service attribute or value does not exist. (Exception from HRESULT: 0x8007200A)

I'm uncertain as to what exactly is wrong. I've search around to find nothing odd about the syntax. Any assistance would be appreciated. Thanks...