It’s hard to tell because the deviating form in Latin is actually the nominative singular, which is why vocab lists include the genitive singular as well. All other forms have the same stem aside from Nom. Sg. A few examples are:

senex - senēs (elder)

rēx - rēgēs (king)

index - indīcēs (index)

So really anything could work as long as it ends on -ēs in plural and starts with kleen-.