Error in downloaded excel of calculation question

Hi,
I have added few calculation questions in a form which are subjected to some conditions.
For example I have asked a person to select their source of income from four different categories (A,B,C,D)… Based on the sources they select, they need to mention income for the particular source and their total income will then get calculated.
Now, if the respondent is selecting all four sources, then they need to enter income for all and there is no problem in calculations visible in form (through a note) and downloaded excel.
However, if the respondent is selecting only a few income sources and mentioning the respective income, then there is no problem in calculations visible in form (through a note) but it is showing error in downloaded excel as ‘Nan’. Please suggest how to resolve this.

@Lannyc, this post discussed previously should help you solve your issue: