  Precise measurements of mass in dark matter dominated dwarf spheroidal
galaxies are of great importance for testing the theories of structure
formation. We use $N$-body simulations of the tidal evolution of a dwarf galaxy
orbiting the Milky Way to generate mock kinematical data sets and use them to
test the reliability of a simple mass estimator proposed by Wolf et al. The
evolution of the initially disky dwarf galaxy embedded in a dark matter halo
was traced for 10 Gyr on a rather tight orbit. After about half of the time a
dwarf spheroidal galaxy is formed that retains some remnant rotation and a
non-spherical shape. Observing the triaxial galaxy along each of its principal
axes we measure its half-light radius and the line-of-sight velocity dispersion
and use them to estimate the mass. We find that the mass is significantly
overestimated when the dwarf is seen along the longest axis of the stellar
component and underestimated when observed along the shortest axis. We provide
a formula that quantifies the systematic error in the estimated mass with
respect to the true one as a function of the galaxy shape and line of sight.
