The third installment of the advance child tax credit hit the bank accounts of most American households with children this week. That’s monthly cash deposits of up to $300 per child that the government started paying in July. Those payments are currently set to expire in December. But a new national survey suggests that some low-income families may not be receiving this money.

Low-income families might not get the advance child tax credit if they’re not required to submit a tax return. 

“Generally, people have to file a tax return for the IRS to know their information,” said Jacob Goldin, an economist and law professor at Stanford.  

Roughly 4 million children and their families might be missing out on the payments for that reason, according to the Center on Budget and Policy Priorities.

Families can still get the credit by signing up online. 

The number of households getting payments each month is going up, “which means that parents are signing and more parents are getting reached as time goes on,” said Ashley Burnside with the Center for Law and Social Policy.

There might be an easier way, going forward, per Goldin. The IRS could use information from other safety-net programs “and use that data to automatically enroll people in the child tax credit, if it looks like they’re eligible to get it,” he said.

Congress is currently taking that up in the budget bill, as well as the future of the child tax credit more broadly.