Parashas Naso · Passuk 6:11
The priest shall offer one as a purgation offering and the other as a burnt offering, and make expiation on [the nazirite’s] behalf for the guilt incurred through the corpse. That same day their head shall be reconsecrated;
Chabad.org — English translation + Rashi
Source: Nazir 45a.
English: “And make atonement for him” — Cutheans have no naziriteship, for Scripture says “and make atonement for him” — one who is eligible for atonement.
In context: The requirement of kapparah excludes those outside the covenant framework of Temple atonement.
Custom
Chazal exclude Cutheans from the institution of naziriteship because they lack the framework of sacrificial atonement.