  Let $\Omega \subset \mathbb{R}^2$ be an open, bounded and Lipschitz set. We
consider the torsion problem for the Laplace operator associated to $\Omega$
with Robin boundary conditions. In this setting, we study the equality case in
the Talenti-type comparison, proved in arXiv:1909.11950. We prove that the
equality is achieved only if $\Omega$ is a disk and the torsion function $u$ is
radial.
