Let E{F be a quadratic extension of fields, and G a connected quasi-split reductive group over F. Let G op be the opposition group obtained by twisting G by the duality involution considered by Prasad. Assume that the field F is finite. Let π be an irreducible generic representation of GpEq. When π is a Shintani base change lift of some representation of G op pFq, we give an explicit nonzero GpFq-invariant vector in terms of the Whittaker vector of π. This shows particularly that π is GpFq-distinguished.When the field F is p-adic, the paper also proves that the duality involution takes an irreducible admissible generic representation of GpFq to its contragredient. As a special case of this result, all generic representations of G 2 , F 4 or E 8 are self-dual.