Scoring and math operations

I am using the scoring function in one of my surveys. I am then sending an email showing the scores for various categories and the survey responses. Most of the scoring categories are either a sum of the individual question scores (Score) or an average of the individual question scores (Weighted Mean). These are all working fine. However, one of my scores requires a more complex mathematical equation. I have used the Math Operations support page to create the mathematical equation. The formula is working and shows the correct score. However, if a respondent does not complete the questions related to that category, the score is showing up at -16.6666667, rather than appearing as 0 or empty. Is there a way to stop this from happening?