  We report measurements of the in-plane electrical resistivity $\rho$ and the
thermal conductivity $\kappa$ of the intercalated graphite superconductor
C$_6$Yb to temperatures as low as $T_c$/100. When a field is applied along the
c-axis, the residual electronic linear term $\kappa_0/T$ evolves in an
exponential manner for $H_{c1} < H < H_{c2}$. This activated behaviour
establishes the order parameter as unambiguously s-wave, and rules out the
possibility of multi-gap or unconventional superconductivity in this system.
