Aggregation not working with other fields except datefields?

[billy-whizz]

From the guide, I can see aggregation being used with date fields. It's not working when I apply it with other integer/decimal fields. For example:

include ReportsKit::Model
  reports_kit do
    aggregation :total_salaries_by author, [:sum, 'authors.salary + staffs.royalty']

Both salary and royalty are decimals.