Greetings! Stupid question, I know, but it was argued ad nauseum at my game today. Hunter's Mercy (Spell Compendium p.117) states that your bow will strike a critical hit. The ranger who cast the spell stated that the damage was automatic - no roll necessary. I counter argued that you do get to score critical damage, but you need to hit the target first, using a d20 to determine success whether you hit or not. I am almost certain that I am right, but I would greatly appreciate if more experienced DMs and players out there could clarify this little spell for me.
Thank you very much in advance.