Signs of the different contributions to the isotope shift of an atomic line are discussed in details to clarify some confusing differences between the electronic parameters calculated with RIS (Nazé et al. Comput. Phys. Commun. 184 (2013) 2187 ; Ekman et al. Comput. Phys. Commun. 235 (2019) 433) and those appearing in other commonly used expressions.