Text (FS+errata): Counter target spell if no mana was spent to cast it. [Oracle 2009/07/08]
It can target any spell. The amount of mana spent is checked only on resolution. [Future Sight FAQ 2007/05/01]
The mana cost may be more than zero as long as an alternate cost or a cost reduction was used to avoid paying it, or if it is a copy of a spell so no mana was pad when playing it. [Future Sight FAQ 2007/05/01]
