  The excitation energy of the 1/2$^-$ isomer in $^{99}$In at ${N=50}$ is
measured to be 671(37) keV and the mass uncertainty of the 9/2$^+$ ground state
is significantly reduced using the ISOLTRAP mass spectrometer at ISOLDE/CERN.
The measurements exploit a major improvement in the resolution of the
multi-reflection time-of-flight mass spectrometer. The results reveal an
intriguing constancy of the $1/2^-$ isomer excitation energies in
neutron-deficient indium that persists down to the $N = 50$ shell closure, even
when all neutrons are removed from the valence shell. This trend is used to
test large-scale shell model, \textit{ab initio}, and density functional theory
calculations. The models have difficulties describing both the isomer
excitation energies and ground-state electromagnetic moments along the indium
chain.
