There are no relationships available to add.

Normally, I should be able to add a rating widget to views results by first adding the relationship and then adding the field. However, this is not working for me because I don't even get the relationship to add - I just get "There are no relationships available to add." Any idea what's up? I don't get any errors or anything, just that notice and no voting api. The view is of type Default node index. Is that possibly the problem?