In Excel, can I use =SUM to total up results of =LOOKUP?

I have made a calorie counting spreadsheet which basically has the calories of certain foodstuffs in a Lookup table. At the end of the row, I use =SUM to calculate the daily calories. This works fine if I have chosen something from every category, but for the categories in which the value is #N/A (because the function is in place, but no choice selected) the result of the sum calculation return #N/A. Is there any solution please?
Cozmosis, you are a genius! Thank you!