  It is shown in this paper that deviations of galaxy cluster lenses from
spherical symmetry can render mass estimates for galaxy clusters based on the
formation of large arcs systematically to high. Numerical models show that the
mass needed for producing large arcs in clusters can be notably smaller than
expected from simple spherically symmetric lens models. The reason is that the
enhanced tidal effect in asymmetric and substructured lenses can compensate for
part of the convergence necessary for strong lensing effects. An analytic
argument is given to explain why deviations from radial symmetry will in
general decrease the required lens mass. Mass estimates assuming radially
symmetric lenses are on average too high by a factor of $\simeq1.6$, and with a
probability of $\simeq20\%$ by a factor of $\simeq2$.
