RSLogix 5000, ADD is not adding

Hello.

A colleague sent me a screenshot (attached) with a peculiar issue. An ADD function that only sends 0 as the outcome, and a DTOS function that only sends " .

I have confirmed that all variables used in the ADD block are declared as integers, and both SumInt6 and String10 are not currently being modified somewhere else. 

This impatient colleague is baffled, while I am just sharing this issue in the hopes that someone will eventually come with the simple answer that we are overlooking, or a complex one that we could never hope to realize by ourselves.

Thanks in advance.