The discrepancies between b →s ℓ+ℓ- data and the corresponding Standard Model predictions constitute one of the most significant hints for new physics currently available. In fact, many scenarios that can account for these anomalies have been proposed in the literature. However, only a single light new physics explanation, i.e., with a mass below the B meson scale, is possible: a light Z' boson. In this article, we aim at excluding any light Z' model as a solution to b →s ℓ+ℓ- data, using a minimal and conservative setup. Considering the improved limits on B → K(*)ν ν , including the experimental sensitivities required for a proper treatment of the necessarily sizable Z' width, together with the forthcoming Belle II analyses of e+e-→μ+μ-+invisible, can rule out a Z' explanation of b →s ℓ+ℓ- data with a mass below ≈4 GeV . Importantly, such a light Z' is the only viable single particle solution to the b →s ℓ+ ℓ- anomalies predicting R (K(*)) >0 in high q2 bins, therefore providing an essential consistency test of data.