OSIS-4287 : Issue 17 : fix sum without none
OSIS-4287 : Issue 17 : fix sum without none

uclouvain/OSIS-Louvain,uclouvain/osis,uclouvain/osis,uclouvain/osis,uclouvain/osis,uclouvain/OSIS-Louvain,uclouvain/OSIS-Louvain