@Anna ********
Not only my experience, some embassies/consulates even mention it in their requirements. Proof of payment is pretty clear. I agree that there is no minimum requirement for said income, but income is pretty much on the menu (meaning unpaid work would not qualify), even if they don't always require a proof of income.
And again, in the cas we are dealing with here, what could this applicant show as a proof of economic activity? Apparently there's no business registered, no contract, etc.