It occasionally happens that a cell looks blank (i.e. null), but it is a blank, not a null. You'll know this is the case because writing if statements using 'is null' don't work. There is a simple fix for this. Instead of writing something like this:
if([PlayName] is null, "Null", "OK")
...write this...
if([PlayName]="", "Null", "OK")
The screen shot shown is using the formula with the quotes to indicate a blank. 