  Anomalies in the flux-ratios of the images of quadruply-lensed quasars have
been used to constrain the nature of dark matter. Assuming these lensing
perturbations are caused by dark matter haloes, it is possible to constrain the
mass of a hypothetical Warm Dark Matter (WDM) particle to be $m_\chi > 5.2$
keV. However, the assumption that perturbations are only caused by DM haloes
might not be correct as other structures, such as filaments and pancakes, exist
and make up a significant fraction of the mass in the universe, ranging between
5$\%$ -- 50$\%$ depending on the dark matter model. Using novel
fragmentation-free simulations of 1 and 3keV WDM cosmologies we study these
"non-halo" structures and estimate their impact on flux-ratio observations. We
find that these structures display sharp density gradients with short
correlation lengths, and can contribute more to the lensing signal than all
haloes up to the half-mode mass combined, thus reducing the differences
expected among WDM models. We estimate that this becomes especially important
for any flux-ratio based constraint sensitive to haloes of mass $M \sim 10^8
M_\odot$. We conclude that accounting for all types structures in
strong-lensing observations is required to improve the accuracy of current and
future constraints.
