  Recent work has suggested that mid-IR wavelengths are optimal for estimating
the mass-to-light ratios of stellar populations and hence the stellar masses of
galaxies. We compare stellar masses deduced from spectral energy distribution
(SED) models, fitted to multi-wavelength optical-NIR photometry, to
luminosities derived from {\it WISE} photometry in the $W1$ and $W2$ bands at
3.6 and 4.5$\mu$m for non-star forming galaxies. The SED derived masses for a
carefully selected sample of low redshift ($z \le 0.15$) passive galaxies agree
with the prediction from stellar population synthesis models that $M_*/L_{W1}
\simeq 0.6$ for all such galaxies, independent of other stellar population
parameters. The small scatter between masses predicted from the optical SED and
from the {\it WISE} measurements implies that random errors (as opposed to
systematic ones such as the use of different IMFs) are smaller than previous,
deliberately conservative, estimates for the SED fits. This test is subtly
different from simultaneously fitting at a wide range of optical and mid-IR
wavelengths, which may just generate a compromise fit: we are directly checking
that the best fit model to the optical data generates an SED whose $M_*/L_{W1}$
is also consistent with separate mid-IR data. We confirm that for passive low
redshift galaxies a fixed $M_*/L_{W1} = 0.65$ can generate masses at least as
accurate as those obtained from more complex methods. Going beyond the mean
value, in agreement with expectations from the models, we see a modest change
in $M_*/L_{W1}$ with SED fitted stellar population age but an insignificant one
with metallicity.
