Network Path not found error when adding a network group to local administrators group

I am trying to add a network group on our domain to the local administrators group on the machine.

I debug line by line and it finds the local administrator group, it finds the network group but when it gets to this line:

AdministratorsGrp.Members.Add(NetworkGrp)

It returns an error "The network path was not found"

How do I fix this error, I spent 30 minutes searching the interwebs and couldn't find a solution :(

Here is my function thus far:

Private Function AddAdminGroup() As Boolean
    Dim AdministratorsGrp As GroupPrincipal
    Dim NetworkGrp As GroupPrincipal

    Try
        Dim DomainCtx As New PrincipalContext(ContextType.Domain, "<domainname>", "<username>", "<pw>")

        'Find Local Administrators group by SID
        AdministratorsGrp = GroupPrincipal.FindByIdentity(DomainCtx, IdentityType.Sid, "S-1-5-32-544")

        'Find Desktop Managers group by Name on Domain
        NetworkGrp = GroupPrincipal.FindByIdentity(DomainCtx, IdentityType.Name, "<network group")

        'Add Desktop Managers group to Administrators Group
        AdministratorsGrp.Members.Add(DesktopMGRGrp)

        'Save Group
        AdministratorsGrp.Save()

        Return True
    Catch PrinEx As PrincipalExistsException
        Return True
    Catch ex As Exception

    End Try
    Return False
End Function

Thanks in advance for any help provided!!

I fixed this problem by changing this line:

Dim DomainCtx As New PrincipalContext(ContextType.Domain, "<domainname>", "<username>", "<pw>")

to

Dim DomainCtx As New PrincipalContext(ContextType.Domain)

I'm not sure why but providing credentials for the domain was causing a problem.