Help with a simple (not to me) ingame /script command

[Garavix]

Okie, first off thanks for looking at the thread. Basically i wanna make a button that casts shadowform, and if shadowform is already up i want it to cast Power Word: Fort. SO heres waht i tried
/script if (not string.find(UnitBuff("player", 1), "Shadowform")) then CastSpellByName("Shadowform"); else CastSpellByName("Power Word: Fortitude");end

What is the index in the unitbuff command supposed to be?

thanks in advance

-Cenarus