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K17 cannot be blank if that formula is returning an error. Based on your earlier post, I'd suspect your formula in those cells is returning " " and not "".
There are many ways to deal with this sort of situation. For example, you can use IFERROR:
=IFERROR(N17*45360*$C$13,"")
which will handle the situation where either N17 or C13 do not have number values in them.
If you know C13 will always have a number in, you could simply test if N17 has a number:
=IF(ISNUMBER(N17),N17*45360*$C$13,"")
You could also amend the formulas in K:N to return 0 rather than "".