Proton capture on the excited isomeric state of Al26 strongly influences the abundance of Mg26 ejected in explosive astronomical events and, as such, plays a critical role in determining the initial content of radiogenic Al26 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 Al26m(p,γ)Si27 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 Mg26 and a possible need to reinvestigate sensitivity studies involving the thermal equilibration of Al26.