  Sijie Gao has recently investigated Hawking radiation from spherically
symmetrical gravitational collapse to an extremal R-N black hole for a real
scalar field. Especially he estimated the upper bound for the expected number
of particles in any wave packet belonging to $\mathcal{H}_{out}$ spontaneously
produced from the state $|0>_{in}$, which confirms the traditional belief that
extremal black holes do not radiate particles. Making some modifications, we
demonstrate that the analysis can go through for a charged scalar field.
