This Array Index is Invalid for This Array - NonMissingMean

Hi,

I'm trying to write a worksheet for some experiments that I carried out. Within the data sets are a number of NaNs, so I've put some code in to find the NonMissingMean, as I have to find the mean of different column vectors. The code is fine for NonMissingMean(D), where D is my data file, but when I try NonMissingMean(D^<2>) i.e. the mean of all data in column 2 that is a number, it says "this array index is not valid for this array".

I'm hoping someone here knows why this is, and possible ways of fixing it.

Thank you very much for your help in advance.