/enchant command block isue

ok so i am getting into command blocks and as you know if you use the enchant command in a command block it you have to be holding the item for it to enchant but you would think that if it fails to enchant then it wouldent send a positive comparator signal but it does so i'm not sure if this is a bug or intentional but it would be a great help if it only tested positive if the enchant was successful because i have a enchant store where it does /enchant @p[r=4,lm=2] 17 1 then it sends a signal if "positive" to another command block with the command /xp -2l @p[r=4] but if the enchant fails it will still test positive and take my 2 levels and trust me i know what i'm doing this should work if it only tests positive from a successful enchant cause i have the same setup for a totalKillCount store where it tests for my total kills and if i have enough it will give me items so if anyone know a way around this please help but i think this is a note worthy bug and should be looked into and fixed thanks for taking the time to read this

tl;dr

Command block outputs signal if /enchant command fails.