  It was recently estimated that the strong isospin-symmetry breaking (ISB)
corrections to the Fermi matrix element in free neutron decay could be of the
order $10^{-4}$, one order of magnitude larger than the na\"{\i}ve estimate
based on the Behrends-Sirlin-Ademollo-Gatto theorem. To investigate this claim,
we derive a general expression of the leading ISB correction to hadronic Fermi
matrix elements, which takes the form of a four-point correlation function in
lattice gauge theory and is straightforward to compute from first principles.
Our formalism paves the way for the first determination of such correction in
the neutron sector with fully-controlled theory uncertainties.
