Klt singularities of horospherical pairs

Let $X$ be a horospherical $G$-variety and let $D$ be an effective $\mathbb{Q}$-divisor of $X$ that is stable under the action of a Borel subgroup $B$ of $G$ and such that $D+K\mathrm{\_}X$ is $\mathbb{Q}$-Cartier. We prove, using Bott-Samelson resolutions, that the pair $(X,D)$ is klt if and only if $\lfloor D\rfloor=0$