  We present an analytic formula for the $O(\alpha_s)$ corrections to the decay
$t\to H^+b$ for nonzero $m_H$ and $m_b$. The Equivalence Theorem is used to
relate these corrections to those for the process $t\to W^+b$. Contrary to
previously published results we find that the effect of the $b$ quark mass is
negligible for a wide range of $m_t$, $m_H$ and $\tan \beta$.
