the requirement is 'income' but there are phase-outs depending on your income and you can't contribute once you reach 70.5 years old
'deferred income' eventually becomes 'earned income' (over the next 4 years) in any event.
see the link below and especially the section called 'on your own'.
https://turbotax.intuit.com/tax-tips/retirement/boost-your-retirement-savings/L3lryQHVz
