Solute effects on high-entropy alloys of equiatomic proportions are scientifically intriguing because there is no such well-defined "solute" and "solvent" atoms compared to those of conventional single principal element alloys. To date, most of the face-centered cubic alloys exhibit solid solution strengthening rather than softening due to the interactions between dislocations and solute atoms. Here, we present the careful experimental measurements and demonstrate solid solution softening, albeit weak, in a single phase CoCrFeMnNi through the minor addition of 2. at.% Al. This softening effect is mostly related to the decreased Peierl's stress by Al addition.