(CNN) A former top adviser to President Richard Nixon said Saturday that while he doesn't think tapes of conversations between President Donald Trump and ex-FBI chief James Comey exist, a "taping device" should always be on hand in the Oval Office.

Pat Buchanan's comments came in response to CNN host Michael Smerconish's question about a tweet Trump sent three days after firing Comey . The President posted that Comey "better hope there are no tapes of our conversations before he starts leaking to the press."

James Comey better hope that there are no "tapes" of our conversations before he starts leaking to the press!

Was the tweet, Smerconish asked Buchanan, a "threat or an admission?"

Neither, Buchanan replied, because he doesn't think such tapes exist.

"I cannot believe the FBI director would walk in the Oval Office and tape the President of the United States," he said.

"My guess is no taping device exists or did not exist at the time of that meeting, and that, there's sort of a warning: Tell the truth, Mr. Comey," Buchanan said.

If there wasn't any taping device, Buchanan said, that was a mistake on the part of the White House, which hasn't been home to a covert recording system since Nixon moved out.

"The President of the United States needs a taping device in the Oval Office," Buchanan said. "If you've got, for example ... the Russians, and what did they say about Syria and Iran and what did they say about Afghanistan? I mean, you want evidence of what they said, and they would expect you would be taping them."

But what about Buchanan's infamous advice to Nixon as Watergate unfolded to "burn the tapes"? Would he repeat it now to Trump? (Note: The Presidential Recordings and Materials Preservation Act of 1974 says tapes like the ones Nixon recorded are presidential records that must be preserved in federal archives.)

"No, if he's made them," replied Buchanan. "Don't you want a tape recording and don't you want a record of that, that you can look over and say, what exactly did they commit to, and what didn't they commit to?" he said.

"Yes," Smerconish replied. "But not between the FBI director and the President."