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Proton capture on the excited isomeric state of 26 Al strongly influences the abundance of 26 Mg ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic 26 Al in presolar grains. This reaction also affects the temperature range for thermal equilibrium between the ground and isomeric levels. We present a novel technique, which exploits the isospin symmetry of the nuclear force, to address the long-standing challenge of determining proton-capture rates on excited nuclear levels. Such a technique has in-built tests that strongly support its veracity and, for the first time, we have experimentally constrained the strengths of resonances that dominate the astrophysical 26m Alðp; γÞ 27 Si reaction. These constraints demonstrate that the rate is at least a factor ∼8 lower than previously expected, indicating an increase in the stellar production of 26 Mg and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of 26 Al.