that's not a dotnetnuke error, it comes from the microsoft memberrole provider. We have our own membership role provider which uses it as the default and then extends it, so to work with roles you need to work with the dotnetnuke API - something like the following should work for you

Dim objRoles As New RoleController
Dim arrRoles As ArrayList = objRoles.GetPortalRoles(PortalID)

Cathal

theres a chm of the API available - see http://www.dotnetnuke.com/Community/Blogs/tabid/825/EntryID/1919/Default.aspx for more. Theres a lot of information that needs to be expanded - i've down a little work to generate some more useful comments, but i'm waiting until after 5.0 to add them. We're also looking to integrate a documentation step into our release procedure so each new release will generate updated API documentation, preferabbly in both downloadable and online fashion (a browsable ajax enabled website for the online version)

Cathal