A preposition ending in i or u lengthens its final before kāśa,
As नीकाशः, वीकाशः, अनूकाशः ॥ काश is formed by अच् (III. I. 134). Why do we say 'ending in इ or उ'? Observe प्रकाशः where the Preposition does not end in इक् ॥,
इकः काशे
Adhyāya 6 · Pāda 3 · Rule 123