firstsortedvalue("field detail", -FileDate)
I get a the - character indicating a null.
The firstsortedvalue function will also return null if there are several "field detail" values for the maximum FileDate (for the dimension values on that row of your chart). Are you sure that this is not the case with your data?
This isn't the case for my data.
The full story is, I'm reading in multiple files, all with dates and file sizes. In a few cases, I'm missing files for 'today'. The firstsortedvalue returns the correct answer in all cases (I've checked with my source data). nullcount returns the correct answer on most occasions, but under some conditions, it returns the wrong answer. Having slept on it and re-thought he whole thing, I'm pretty sure this is a bug.