Are there any funny limitations on the cast function?

This crashes:
CODE
        set x to "player"->getpos x
        set y to "player"->getpos y
        set z to "player"->getpos z
        set z to z + 20
        aaaFireStormSource->position,x,y,z,0
        aaaFireStormSource->Cast aaaFireStormDamage player


x,y,and z are floats, aaaFireStormSource is an activator, and aaaFireStormDamage is a fireball spell.