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I am using ArcGIS Pro, field calculator to add the values of two fields (Result_A and Result_B) and assign the value to new Field Combined_Result. The type for all three fields is "double". Both fields for Result_A and Result_B have some cells with null values. This is generating errors in their addition. In the Calculate Field window (for field Combined_Result) For the calculate field, I am using the following expresssion:

=!Result_A! + !Result_B!

I get the following error:

WARNING 002858: Certain rows set to NULL due to error while evaluating python expression: TypeError: unsupported operand type(s) for +: 'float' and 'NoneType'

I have tried changing the fields to float but get the same error.

Should I convert null values to zero before running the expression?

I have beginner's level python skills and can set up a code to convert null values if you point me in the right direction.

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I did not have to convert the null value cells to zero. Instead I wrote the following code block in the "Calculate Field" window for Combined_Result

= ((!Result_A! or 0) + (!Result_B! or 0))

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