A guttural is not substituted for the palatal before the affix ṇya when it means 'to do as absolute necessity'.,
As अवश्य पाच्यम्, अवश्य वाच्यम्, अवश्यरेच्यम्, but पाक्यं, वाक्यं and रेक्यं when the sense is not that of necessity.,
ण्य आवश्यके
Adhyāya 7 · Pāda 3 · Rule 65