Assign Null value to Variable

I have a column with int data type.. i am trying to assign this column to a variable ( int) in For Each Loop..but it keeps giving me an erro

The type of the value being assigned to variable "User:ubcontractor_Key" differs from the current variable type. Variables may not change type during execution. Variable types are strict, except for variables of type Object.

Am i getting this error becasue the column has NULL value?. how can I resove this probelm?

Populate your source with 0s instead of NULLs in your source query, if applicable. (Or some arbitrary number)|||

can i do thsi in Expression builder using NULL function?

if so, can you show me some examples?

|||Anywhere in an expression builder, you can do:

ISNULL(ColumnOrVariable) ? 0 : ColumnOrVariable