American workers are cashing in on their side hustle, picking up small jobs through Task Rabbit, driving for Lyft, or selling jewelry on Etsy. But they owe taxes on that work. And many of these self-employed workers are under-reporting their income to the IRS. It’s contributing to an estimated $450 billion tax gap.

Annette Nellen, who directs the graduate tax program at San Jose State University, spoke with Bloomberg Tax reporter Amanda Iacone about how the gig economy is changing the tax landscape and the challenges facing both workers and preparers.