The affix śnu comes after the roots of svādi class when a sārvadhātuka affix denoting an agent follows.,
This debars शप्. As सु + श्नु + तिप् = सु + नु + ति = सुनोति 'he presses out'. These are roots of the Fifth class.,
स्व्-आदिभ्यः श्नुः
Adhyāya 3 · Pāda 1 · Rule 73