Sen. Bernie Sanders promised to release 10 years of his tax returns, like fellow 2020 Democratic candidate Elizabeth Warren has.

He did not, however, give a timeline for when he could release them.

"Soon," he told CNN's Wolf Blitzer.

Then he added: "They're very boring tax returns."

Some context:

Some Democrats have criticized Sanders for not being more forthcoming with his financial information during the 2016 primary, when he only made public his returns from 2014.

President Donald Trump's refusal to release his tax returns, a departure from more than 40 years of tradition, has riled Democrats and ethics watchdogs who worry he could be concealing assets in an effort to deflect from potential conflicts of interest. Massachusetts Sen. Elizabeth Warren has led the way in calling for more transparency from the President and her fellow Democratic candidates.

"I put the past 10 years of my federal tax returns online. And now I'm calling on every other candidate for President to do exactly the same thing," Warren tweeted earlier this month. She has also posted an online petition calling "on everyone running for president to release their tax returns."

Hillary Clinton challenged Sanders to release his returns during a primary debate in April 2015.

"They are very boring tax returns. No big money from speeches, no major investments," he said then, much like he did on Monday night.

"Unfortunately, I remain one of the poorer members of the United States Senate. And that's what that will show."

According to the most recent public records, Sanders ranks among the lowest earners on Capitol Hill. His net worth is in the bottom dozen of senators from both parties and the smallest of the 2020 Democratic primary candidates.

A day later, Sanders released documents showing that he and his wife, Jane, earned nearly $206,000 the year before, when they paid about $28,000 in federal taxes. They also collected about $46,000 in Social Security benefits.

The lion's share of the couple's income in 2014 came from Sanders' Senate salary, which was $174,000. They have more than $8,000 in gifts.