We determine the decay rate Γ of a scalar particle bound to a light nucleus, numerically and analytically. As a model system, we consider a negatively charged kaon in the ground state of a hydrogen-like ion decaying into charged and neutral scalar pions, K− → π−π0. All particles are assumed point-like; the nucleus is infinitely heavy. The partial wave expansion converges slowly, as in a recent analysis of the bound-muon decay. To overcome this difficulty, we introduce a subtraction method: the free π− contribution is evaluated separately, while the partial-wave sum is used only for the Coulomb distortion. The ratio of bound-to-free decay rates, Γ/Γ0, is found for light nuclei, 1 ≤ Z ≤ 10. The numerical calculation is complemented by a diagrammatic small-αZ expansion. We find Γ/Γ0 = 1 − (αZ)2/2 + 17(αZ)4/24 + O((αZ)5), in agreement with the all-orders numerical calculation at low Z.