Edward Yardeni is questioning the market's win streak.
The long-time bull believes the record rally will take a breather because valuation multiples are getting back to nosebleed altitudes.
"I'm concerned about a possible melt-up here," the Yardeni Research president told CNBC's "Trading Nation" on Friday. "I've been shooting for 3,500 for the S&P 500 by the end of next year, and we're getting closer. Faster than I would have expected."
And the consequences could be downright painful.
"[A] 10% to 20% [correction] would be quite possible if this market gets to 3,500 well ahead of my schedule," he said.
Yardeni said he has been worrying about the rally for a couple of months.
During "Trading Nation" on Nov. 1, Yardeni said: "I just don't want too much of a good thing here. I'd like this bull market to continue at a leisurely pace, not in a melt-up fashion."
Since then, the S&P 500 is up almost 7% while the Nasdaq has surged about 9%.
"Bull markets do best when you've got a wall of worries," Yardeni said. "What I'm worrying about is nobody is worried anymore."
But Yardeni, who's known for running investment strategy for Prudential and Deutsche Bank, still considers himself a bull. His 2020 S&P 500 target is tied for highest on Wall Street. Plus, he sees no recession through 2021.
"Looking at 2020, I'm expecting earnings to be up 4% to 5%, which isn't fabulous," he said. "But it should be enough to get the market up to 3,500 by the end of next year."
Without a strong economy and rapidly growing earnings, Yardeni notes it's hard to see smooth sailing for the rally.
He said he'd be hesitant to put new money to work in U.S. stocks right now. According to Yardeni, the safest bet is to wait for the next pullback. His strategy: Pick up stocks, particularly in the technology space, at a better value.
"This is not a cheap market," Yardeni said. "In early October, I looked around and said 'you know, maybe there's some value overseas. So maybe you really got to look at emerging markets.'"