  Narison's $\tau$-decay-like sum rule for determining the strange quark mass
is re-investigated, taking into account isospin-breaking corrections in the
extraction of the input spectral functions from $e^+e^-\to hadrons$ data. The
corrections, estimated using experimental data on vector meson electromagnetic
decay constants and a QCD sum rule analysis of the 38 vector current
correlator, are shown to be especially large for the isoscalar case. The reason
such large corrections are natural is also explained. Due to the high degree of
cancellation in the original sum rule, the effect of these corrections on the
determination of $m_s$ is significant. A new central value $m_s=113-138 MeV$ is
found, in the $\bar{MS}$ scheme at 1 GeV$^2$, with significant (asymmetric)
errors associated with errors in the input experimental data.
