How do I resolve warning 002858?

[KatieMichel]

Hi, I'm trying to find the percent change of temperature using two climate projections with the formula %change=((new-old)/old)*100. When running it through "calculate field" this is what I have:

and I get this warning message with all of my values returned as null:

Any ideas? 

[DanPatterson]

It is a warning... one of your two fields has a null value in it, so when it tries to perform the numerator operation, it is either getting

a number - None

or

None - a number

So you will have to fix your data if you aren't expecting nulls in those fields, or you will have to fix the expression to deal with nulls.

In any event, a warning probably didn't make all the calculations fail did it?

[IssamAchour]

yeah how to perform calculations and you have NULL values? i have these kinds of data since this information aren't reported so it must be involved in my geoprocessing work

 how to deal with that using python?