Joe Biden’s Democratic presidential campaign amended his climate policy plan hours after it was released Tuesday because a handful of passages did not credit some of the sources in the proposal.

The changes come after The Daily Caller, a conservative news outlet, reported that several passages from Biden’s plan appeared to borrow from policy papers and statements written by outside groups without citation.

A Biden campaign statement says, “Several citations, some from sources cited in other parts of the plan, were inadvertently left out of the final version of the 22-page document.”

The added citations included passages from documents published by the Blue Green Alliance and the Carbon Capture Coalition. The document by Biden, a former vice president under Barack Obama, cites sources in the text of the plan. Some other campaigns use more formal footnotes in their policy papers.

Biden’s 1988 presidential campaign ended amid controversy over plagiarized passages in his campaign speech and after he admitted plagiarism on a law school paper. At the time, Biden said the incident at Syracuse University was a matter of him not knowing the proper way to document sources.

David Axelrod, a Democratic strategist and architect of Obama’s 2008 campaign, was skeptical that the errors in his climate policy proposal will affect his front-running campaign.

“There are many, many molehills that look like mountains during a campaign,” Axelrod said. “The challenge for the campaigns and the press is to decide which is which.”