I got some problems with saving/restoring my equipment sets with this addon. In process of pinpointing issue after some deleting and recreating new sets, problem with saving/restoring went away for some reason. Nonetheless, it seems as equipment sets are indexed starting from 0 to GetNumEquipmentSets() - 1. For example:
/run for i = 0, C_EquipmentSet.GetNumEquipmentSets()-1 do n, icon, index=C_EquipmentSet.GetEquipmentSetInfo(i);print(n,icon,index," -- ",i) end
Gives correct output:

Feral 132402 0  --  0
Heal 132378 1  --  1
test 254105 2  --  2

/run for i = 1, C_EquipmentSet.GetNumEquipmentSets() do n, icon, index=C_EquipmentSet.GetEquipmentSetInfo(i);print(n,icon,index," -- ",i) end
Does not:

Heal 132378 0  --  1
test 254105 1  --  2
nil nil nil  --  3

Not sure if i referenced right line to correct, as it seems I am unable to recreate issue anymore.

i think it is ok to enumerate from 0
myslot will check if get(i) == expected

fixed 5.4.2