Xanthoproteic test for phenylalanine

I read on the net that phenylalanine gives a negative result with xanthoproteic test and the reason that was given was that the benzene ring is deactivated and hence the extent of nitration is very less. 


What I dont understand is that isn't the benzene ring in phenylalanine actually activated through hyperconjugation from the $\ce{CH2}$ group of $\beta$-carbon and hence shouldn't it give positive results with the xanthoproteic test?


