What are you getting in the cells without values? #N/A, or some such error message? If so, you can wrap your current formula in an IFERROR formula, where you can specify what should be shown/returned in cells that evaluate to an error. Syntax is =IFERROR(your formula here,value if error) - I suggest a "0" if error, so that your sum formula will still work.
Hope this helps.
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