I think the fifth is interpreted more broadly than that. You don't need to have a specific law in mind that you believe you have broken (and would reveal) to take the fifth; you merely need to believe that it is possibly that what you say might be incriminating in some way. Which, given the fact that no person could reasonably be expected to know the whole of the law, and given that lying to a federal agent in the performance of their duties is always illegal, is a pretty safe bet.
