  The rare decay $B^0 \to K^{\ast 0}\ell^+\ell^-$ is a flavour changing neutral
current decay with a high sensitivity to physics beyond the Standard Model.
Nearly all theoretical predictions and all experimental measurements so far
have assumed a $K^{\ast 0}$ P-wave that decays into the $K^+\pi^-$ final state.
In this paper the addition of an S-wave within the $K^+\pi^-$ system of $B^0
\to K^{\ast 0}\ell^+\ell^-$ and the subsequent impact of this on the angular
distribution of the final state particles is explored. The inclusion of the
S-wave causes a distinction between the values of the angular observables
obtained from counting experiments and those obtained from fits to the angular
distribution. The effect of a non-zero S-wave on an angular analysis of $B^0
\to K^{\ast 0}\ell^+\ell^-$ is assessed as a function of dataset size and the
relative size of the S-wave amplitude. An S-wave contribution, equivalent to
what is measured in $B^0 \to J/\psi K^{\ast 0}$ at BaBar, leads to a
significant bias on the angular observables for datasets of above 200 signal
decays. Any future experimental analysis of the $K^+\pi^-\ell^+\ell^-$ final
state will have to take the S-wave contribution into account.
