Gavin,
I tried
=AND(B2>=0,B2<=15,ISBLANK(C2))
and still do not get an error message when I enter a value such as 25 in cell B2 and C2 is blank.
I also tried
=AND(B2>=0,B2<=15,LEN(C2)=0)
and again do not get an error message when I enter a value such as 25 in cell B2 and C2 is blank.
any...