how to get basic formula to work in column D when there is already a formula in column C

Hi

I'm trying to get this formula in column D to work =IF(C4="","",SUM(C4-B4)), but there is already a formula giving a total in column C. If I remove this formula in column C the =IF(C4="","",SUM(C4-B4)) formula works. I've even tried the formula =IF(C4="0","0",SUM(C4-B4)) but still it won't work.

How can I get the formula =IF(C4="","",SUM(C4-B4)) to work with the formula in column C so I get a true picture of what I'm trying to count.

The formula in column C brings in totals from various tabs as does column B

If Im reading it correctly then the issue is with the dat atype, if theres a calculation in C then C must be a numeric value therefore:

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Should do it.