Excel does not let me select more than 14 non continuous cells for an average formula

I'm trying to get an average of certain cells that are not continuous (E5, E8, E11 . . .) in a columns. The problem is that when I hold down ctrl and select the cells by clicking on them, the selections come off once I select the 15th cell.

I can't figure out why this is happening. And it's happening in only one workbook.

I just did a test with a new worksheet and it did not do this.


Any ideas?


Excel 20110 Home and Office