The possibility of superconductivity in tetragonal FeS has attracted considerable interest because of its similarities to the FeSe superconductor. However, for a long time all efforts made to prepare tetragonal FeS have failed, and it was not known whether tetragonal FeS is metallic or semiconducting. In 2015, a new synthesis route was identified and FeS was found to be a 5 K superconductor. Comparison with FeSe demonstrates that the two materials are quite different and the change from sulphur to selenium is associated with some rather subtle changes in physical behaviour. We propose to measure the superfluid stiffness in this newly discovered superconductor and to use the resulting information to compare with data previously obtained on FeSe.