These jobs aren’t taxing.

About 1 in 4 Americans now has a side gig, according to a study of 2,245 American adults, commissioned by personal finance site Finder.com and carried out by global research provider Pureprofile. And that’s a problem for the IRS because people with side hustles are earning an estimated $214.6 billion that they aren’t declaring to the tax man, the researchers estimate.

While this is just an estimate — and a rough one at that — it’s clear that plenty of people are doing things like dog walking/pet sitting, selling crafts or goodies, house cleaning, babysitting, tutoring, handyman work, gardening and more, often for undeclared cash. More than one in three millennials has a side hustle — the highest of any of the generations studied.

“Side gigs are becoming more popular and it’s no surprise that so many Americans are showing their entrepreneurial spirit and jumping on board,” says Finder.com’s consumer advocate, Jennifer McDermott. “What is surprising is the number of those making cash ‘under the table’. If you’re a side gig professional – or just jumping on this trend – it’s important to remember that in most cases, you still have to report your income to Uncle Sam.”

Also see: 1 in 3 Americans are this frighteningly close to plummeting into poverty

If you don’t declare your side gig income, the IRS can penalize you, as it is considered tax fraud not to report it in many cases. In general, if you earn more than a certain amount from all sources of income — for a single person under 65 it was $10,350 in 2016  — you must file a tax return to declare your sources of income like side gigs and pay taxes on that income.

If you don’t and are caught, you can face these penalties. There are penalties for things like underpayment of taxes, nonpayment of taxes and for failing to file at all. Not filing at all will cost you a penalty of 5% of your unpaid tax bill each month, not paying what you owe will be 0.5% each month, and you can find out more about the underpayment rules here. Of the estimated $458 billion that the IRS loses to tax evasion, it estimates that it will ultimately recover about $52 billion of that.

Many people earning money in a side gig have no idea how to file taxes around that extra income. This guide from Policy Genius shows you how to do your taxes when you have a full-time job and a side gig. One particularly helpful hint: You’ll need to keep good records throughout the year so you can calculate earnings, deductions, expenses and more.