Integrate Variable filter does not produce my expected result!

Say I am using ParaView to visualize/post-process results of a 3D simulation where the case name is foo.foam with imported cell arrays: V (volume of cell), and X (some arbitrary scalar variable).
So, I apply the Integrate Variable filter to the foo.foam; then I go to the spreadsheet and set attribute to Cell type. Among all the data in the single-row sheet, I see a column called Volume which shows the total volume of my domain. So far, so good. The Integrate Variable filter is producing the expected result.
Next, I apply a Calculator filter to the foo.foam with attribute set to Cell and name set to Sample and enter the expression (V * X) into the Calculator. Now, if I go to the spreadsheet, I see a column for all the cells in my domain with the title Sample and the corresponding value (result of V * X) for each cell. So far, so good.
Now, I need to sum (integrate) this value (V * X) for the entire domain. I apply the Integrate Variable filter on the Sample, assuming I will get a column in the single-row spreadsheet that gives me the sum of V*X for the entire domain. However, no new column is being created.
How do I get value of the “sum” I need?