We discuss the impact of the uncertainty ($pm 8$ keV) in the excitation energy of the astrophysically important 6.356 MeV $1/2^+_2$ state of $^{17}$O on the precision with which the Coulomb reduced ANC ($widetilde{C}$) for the $left<^{17}mathrm{O}(1/2^+_2) mid protect{^{13}mathrm{C}} + alpha right>$ overlap can be extracted from direct reaction data. We find a linear dependence of $widetilde{C}^2$ on the binding energy, the value extracted varying by a factor of 4 over the range $E_{mathrm{ex}} = 6.356$ -- $6.348$ MeV. This represents an intrinsic limit on the precision with which $widetilde{C}^2$ can be determined which cannot be improved unless or until the uncertainty in $E_{mathrm{ex}}$ is reduced.
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