Summing a column of figures defaults to date format

I am suddenly encountering a bizarre behaviour in numbers, since I updated to the latest version. If I try to sum a column of figures, whether formatted as number or as currency, the formula result is displayed as a date. If I try to reformat the cell to number, the cell appears blank. I have tried deleting the offending sum cell, redoing the column of figures and summing those, formatting all cells to currency or number before entering the formula, and reformatting after entering the formula. Nothing seems to work. I can't get a simple result of a sum in a number or currency format. Has anyone else encountered this strage behaviour in the latest numbers update? If so, what is the fix?