Since this is an array formula, you must press Ctrl+Shift+Enter
rather than just Enter when you first enter the formula and
whenever you edit it later. If you do this properly, Excel will
display the formula enclosed in curly braces {}.
Well you need not worry about null (blank) cell values as they will be
ignored by the AVERAGE function. Zero's on the otherhand, will give
you skewed results - if they are not values to be Averaged. You have
many options, here is one ...