First, are you sure you have a cassette and not a freewheel? The Shimano cassette lockring remover tool (Park FR5) and the Shimano freewheel remover (Park FR1) are not the same and are not interchangable. They are similar in appearance but don't interchange.
Note to Bezalel: Mudpie is correct that Park made a running change to the BBT-2 tool to let it clear Octalink and ISIS spindles but didn't change the model number. The only way to tell was that the blister pack on the revised BBT-2 had a yellow sticker saying it now fit larger diameter spindles.