This can occur when the employee has a time entry with a duration of zero. You won't be able to see a zero duration time entry on the Time Entry tab, but you can use the Time Summary tab to find and delete this entry.
On Time Summary, click Options to get the Employee selection box.
Select the employee and week affected, and then double-click on the time entry line. This will bring up a new box which will show all the employee's time entries for the week. Find the line with zero quantity and click Remove to delete line.
Check Time Entry again to ensure that the problem has been solved.