Yaakov bar Zavdi/ R. Yochanan citing R. Simon: Any Maaser Sheni that is not worth a peruta does not need 1/5th added to it.

R. Yochanan citing R. Yanai: Any Maaser Sheni for which the extra 1/5th would be less than a peruta does not need 1/5th added to it.

There are tanaim that supports each of the last two laws...

Beraisa #1: "'From its maaser' - this excludes if it was less than a peruta. This teaches that the Maaser Sheni itself must be worth a peruta.

Beraisa #2: "'From its maaser, its fifth...'' - this excludes less than a peruta. This teaches that its fifth must be a peruta.

Ravin bar Memaleh: There is a Mishnah (taught above) that seems to be like neither of these - 'There are five cases that require a minimum value of a peruta' - and the Mishnah did not teach that the Maaser itself is not worth a peruta; 'There are five cases where a fifth is also paid' - and the Mishnah did not teach that Maaser Sheni's fifth must be a peruta.