  The decay $B^0\to \psi(2S) K^+\pi^-$ is analyzed using $\rm 3~fb^{-1}$ of
$pp$ collision data collected with the LHCb detector. A model-independent
description of the $\psi(2S) \pi$ mass spectrum is obtained, using as input the
$K\pi$ mass spectrum and angular distribution derived directly from data,
without requiring a theoretical description of resonance shapes or their
interference. The hypothesis that the $\psi(2S)\pi$ mass spectrum can be
described in terms of $K\pi$ reflections alone is rejected with more than
8$\sigma$ significance. This provides confirmation, in a model-independent way,
of the need for an additional resonant component in the mass region of the
$Z(4430)^-$ exotic state.
