Only if the sword is protected by its wielder's Magic Resistance. According to "The Functioning of Magic Resistance" on p. 85 of ArM5, it doesn't look like a sword would normally benefit from that protection. Only if it were the wielder's Talisman or something. Normally, it looks to me like your ward would Just Work (TM).
That would have to penetrate vs. any magus who had either a Parma Magica or any other magical effect (such as a Longevity Ritual or any personal range Corpus spells).
I agree with that. Having no Magic Resistance is very different from having a Magic Resistance of zero. No Magic Resistance means, effectively, a Magic Resistance of minus infinity. That's what mundanes have.