  We derive constraints on the couplings of light vector particles to all
first-generation Standard Model fermions using leptonic decays of the charged
pion, $\pi^+\to e^+ \nu_e X_\mu$. In models where the net charge to which
$X_\mu$ couples is not conserved, no lepton helicity flip is required for the
decay to happen, enhancing the decay rate by factors of
${O}(m_\pi^4/m_e^2m_X^2)$. A past search at the SINDRUM-I spectrometer severely
constrains this possibility. In the context of the hypothesized $17$ MeV
particle proposed to explain anomalous $^8$Be, $^4$He, and $^{12}$C nuclear
transitions claimed by the ATOMKI experiment, this limit rules out vector-boson
explanations and poses strong limits on axial-vector ones.
