  We study the shift in the $\Upsilon$ mass due to a non-zero charm quark mass.
This shift affects the value of the $\bar{\rm MS}$ $b$-quark mass extracted
from the $\Upsilon$ system by about -20 MeV, due to an incomplete cancellation
of terms that are non-analytic in the charm quark mass. The precise size of the
shift depends on unknown higher order corrections, and might have a
considerable uncertainty if they are large.
