Self-dual representations with vectors fixed under an Iwahori subgroup

Let $G$ be the group of $F$-points of a split connected reductive $F$-group over a non-Archimedean local field $F$ of characteristic 0. Let $\pi$ be an irreducible smooth self-dual representation of $G$. The space $W$ of $\pi$ carries a non-degenerate $G$-invariant bilinear form $(,)$ which is unique up to scaling. The form is easily seen to be symmetric or skew-symmetric and we set $\epsilon (\pi )=\pm 1$ accordingly. In this article, we show that $\epsilon (\pi )=1$ when $\pi$ is a generic representation of $G$ with non-zero vectors fixed under an Iwahori subgroup $I$.