If I can't use this against zone attacks, why can I use it against single-target attacks?
I'm not so sure about your definition of defence rolls, but regardless; you should make clear whether this can be used against manuevers.
Well, apparently my definition was wrong, according to the glossary. So then, I'd rewrite to say that when defending against an attack, roll vs. the control. When using this against any other type of magical effect, roll against the power of the effect. Good catch.
I have no problems with it affecting zone wide attacks, but I wouldn't want it to shut down the entire attack. "If used against a zone-wide attack, it prevents the attack from affecting you but does not mitigate the entire effect."