  Using the same lens galaxies, the ratios of tangential shears for different
source galaxy redshifts is equal to the ratios of their corresponding
angular-diameter distances. This is the so-called shear-ratio test (SRT) and it
is valid when effects induced by the intervening large-scale structure (LSS)
can be neglected. The dominant LSS effect is magnification bias which, on the
one hand, induces an additional shear, and on the other hand, causes a
magnification of the lens population. Our objective is to quantify the
magnification bias for the SRT and show an easy-to-apply mitigation strategy
that does not rely on additional observations. We use ray-tracing data through
the Millennium simulation to measure the influence of magnification on the SRT
and test our mitigation strategy. Using the SRT as a null-test we find
deviations from zero up to $10 \%$ for a flux-limited sample of lens galaxies,
which is a strong function of lens redshift and the lens-source line-of-sight
separation. Using our mitigation strategy we can improve the null-test by a
factor of $\sim \!100$.
