The nonprofit foundation run by Democratic presidential frontrunner Hillary Clinton and her family admitted for the first time Sunday that it "made mistakes" on how it listed government donors on its IRS filings and said it is working to ensure it doesn't happen again.

"We made mistakes, as many organizations of our size do, but we are acting quickly to remedy them," wrote the foundation's acting CEO Maura Pally. "And [we] have taken steps to ensure they don't happen in the future."

As Reuters reported last week, the errors were found on the tax forms 990 that all nonprofit organizations are required to file annually with the IRS to keep their tax-exempt status. The foundation under-reported and over-reported, by millions of dollars, donations from foreign governments, and for three years in a row, the foundation reported that it received no donations from foreign governments, according to Reuters.

"We have said that after a voluntary external review is completed we will likely refile forms for some years," Pally wrote. "While some have suggested that this indicates a failure to accurately report our total revenue, that is not the case. Our total revenue was accurately reported on each year's form - our error was that government grants were mistakenly combined with other donations. Those same grants have always been properly listed and broken out and available for anyone to see on our audited financial statements, posted on our website."

Pally also defended the foundation's disclosure of controversial contributions from a Canadian billionaire involved in the mining industry.

Allegations have been made in the past week that the foundation accepted millions in donations from Frank Giustra in exchange for Hillary Clinton using her State Department to approve a Russian takeover of one of America's largest uranium mines.

"Like every contributor to the Foundation, the Clinton Giustra Enterprise Partnership (Canada) is publicly listed as a donor on our website," Pally wrote. "But as it is a distinct Canadian organization, separate from the Clinton Foundation, its individual donors are not listed on the site. This is hardly an effort on our part to avoid transparency - unlike in the U.S., under Canadian law; all charities are prohibited from disclosing individual donors without prior permission from each donor."

Pally said the foundation's policies for donor disclosures and foreign government contributors are "stronger than ever," adding that the foundation took "unprecedented steps" to avoid conflicts of interest with donor disclosures when Clinton became secretary of state.

Now that Clinton is running for president, Pally said the foundation will continue to "only accept funding from a handful of governments, many of whom the Foundation receives multi-year grants from, to continue the work they have long partnered on."

"We are committed to operating the foundation responsibly and effectively to continue the life-changing work that this philanthropy is doing every day," Pally wrote.