A comment on "weak phase $\gamma$ using isospin analysis and time-dependent asymmetry in $B^0\to K_S\pi^+\pi^-"

We comment on a recent suggestion for measuring $\gamma$ in $B^0(t)\to K_S\pi^+\pi^-$. A difficulty is pointed out in relating electroweak penguin and tree amplitudes in $B^+\to K^0\pi^+\pi^0$, which is crucial for an implementation of this method. We clarify the necessary condition for such a relation.