Conditional Formatting - Blank Cell Gets Unwanted Format.

As a pert of a spreadsheet I have 2 columns (M5:N400) of 400 cells deep. Each cell in these columns contains a formula that counts days, when appropriate. They can end up showing a value of blank, a number between 0 and 5, or any number greater than 5. (But very rarely greater than 30)

If the number is greater than 5 then it means that it has exceeded it's target time and I wish to highlight this with conditional formatting (CF). I have a CF on all cells in both columns that says.

If "The cell value" is "Greater than" "5"... then
format the background colour to a pale yellow.

It works, but doesn't work.

If the cell value is 0 to 5 it's unchanged. Good.
If the cell value is >5 it changes. Good.
BUT, if the cell is blank it also changes. Bad!!!

I assume it is because it is looking at the fact that there is a formula in the cell, even though the displayed value is "". So I tried a second level saying that if the cell is equal to "" then no format, but it still changes.