Jimmy Fallon took a lot of heat for his unserious interview with Donald Trump earlier this year, but another comedian set him straight on Fallon’s own show.

The host of HBO’s “Real Time,” Bill Maher, lived up to his image as a no-nonsense, foulmouthed political comedian and commentator when he joined Fallon on Monday night’s “Tonight Show.”

Maher tried to make some political points, saying that while he doesn’t think much of Hillary Clinton’s candidacy (“She campaigns like a hospice nurse”) he believes Fallon’s young audience needs to know that this presidential election is unique – and not in a good way, from his point of view.

“This has never happened before, this kind of an election,” Maher said.

He said that Russia hacked Democratic Party organizations, but that FBI is “with Russia.”

Fallon tried to take things in a more lighthearted direction, but when the interview turned to giggles, Maher quickly shut him down.

"It's not funny," Maher said.

Fallon quickly agreed and shut his mouth.

Maher mentioned the "mea culpa" he made on "Real Time" recently, when he said Democrats, including himself, made a "big mistake" when they "cried wolf" on other Republicans who were, in retrospect, more tempered than Trump, such as Mitt Romney or John McCain.

"It would never have been the end of the world with those guys. This is different. This guy is different. They have no respect for the rule of law," Maher said of the Trump campaign on Monday.

Maher also saved some time to slam undecided voters, telling Fallon what he would tell that group.

"I would say, 'Really? I mean, he's a racist, a liar, a tax cheat, a deadbeat, a Russian agent, and a sexual predator. How do you find your way out of the door in the morning?'" Maher said.

Watch the video below: