Chance to block spell damage craft on chests is listed under suffixes, but apparently is a prefix

As the title says, the chance to block spell damage craft on chests is listed under suffixes, but when I tried to place it on my chestpiece with 3 prefixes and open suffixes, i was told it can't have any more prefixes. Really frustrating, since I spent several ex setting up new chestpiece prefixes just so i could have this craft as a suffix.

It'd be nice if this could be cleared up one way or the other, either make it a suffix properly or list it properly under prefixes.