U.S. Chief Justice John Roberts, left, administers the oath of office to President Barack Obama a second time in the Map Room of the White House on Wednesday night. (Pete Souza/White House/Getty Images)

Joking that it was so much fun he had to do it again, Barack Obama became the third U.S. president to take the oath of office a second time because of concerns over the initial swearing-in.

U.S. Supreme Court Chief Justice John Roberts again delivered the oath to Obama on Wednesday night in the White House Map Room, a day after the president took it in front of more than 1.5 million people on the steps of the Capitol in Washington.

Several of the president's aides, a handful of reporters and a White House photographer attended the brief ceremony. No television camera crews or news photographers were present, but the ceremony was audiotaped.

Obama didn't use a Bible for the second oath, but it is still binding.

Roberts mixed up the order of wording of the oath on Tuesday, causing Obama to pause during the ceremony as the judge repeated the phrase.

White House lawyers said the oath was re-administered on Wednesday in "an abundance of caution" to head off any future questions surrounding the legality of the swearing-in.

The U.S. Constitution is clear about the exact wording of the oath, say experts, who said the second swearing-in likely was unnecessary but couldn't hurt.

When Roberts asked Obama whether he was ready to retake the oath, the president replied: "Yes, I am and we're going to do it slowly."

Following the 25-second oath, Roberts offered his congratulations.

"Thank you sir. All right. The bad news for the [reporters] is there's 12 more balls," said Obama, referring to the 10 inaugural balls he and his wife, Michelle, attended Tuesday night.

Obama joins Coolidge, Arthur

During Tuesday's inauguration ceremony, Roberts said: "that I will execute the office of president to the United States faithfully."

The correct wording is "that I will faithfully execute the office of president of the United States."

Obama joins Chester Arthur and Calvin Coolidge as U.S. presidents who have had to retake the oath of office because of unusual circumstances.

Arthur, who served from 1881 to 1885, was sworn in by the chief justice of the New York Supreme Court at his home in a private ceremony following the assassination of former president James Garfield. Arthur was sworn in a second time by the chief justice of the U.S. Supreme Court two days later at the Capitol.

In Coolidge's case, he took the oath of office at his father's Vermont home following the death of former president Warren Harding. Coolidge's father was a justice of the peace and administered the oath. Concerns about the jurisdiction of Coolidge's father led to Coolidge taking a second oath later in Washington.

With files from the Associated Press