BUG in EXCEL/ @SUM, autocorrect current version(s) of Office 365
previous versions of EXCEL allowed a user to input @sum(a1.a10) and excel would autocorrect the . to a : (colon) - making the formula correct and work. This trained excel users that inputting a "." was OK.
Now, Office 365 does NOT autocorrect, throwing a wrench in the gears for users that are accustomed to being able to input a "." in the typed in range