|Excel is notorious for locking a cell into Text format and never letting go. When that happens, it doesn't see your formula as a formula, but as a text string.|
Try this (no guarantees):
Select an empty cell and Copy it
Select the cell that contains =A11 and do an Edit...PasteSpecial...Add
Either this will solve the problem or you'll get a #VALUE error.
If you get a #VALUE error, look in the formula bar and you should see something like:
Delete the +0 and see if that helps.
What this should (hopefully) do is force Excel to apply a number format to the cell, removing the Text format and allow it to accept a formula.
Like I said, no guarantees.