Long-time bull Jeremy Siegel believes the Dow could hit the 30,000 milestone this month.
But he doubts the celebration would last long.
With just 10 trading days left in January, the Wharton Finance professor is warning investors Dow 30,000 would likely be the breaking point for the record rally.
"It's going up a little too far, too fast," he told CNBC's "Trading Nation" on Thursday. "Once you go too fast, a little stone can throw you off."
Siegel's major issue: Stocks are getting way too expensive.
"We have a lot of momentum players in this market. There are a lot of players that are just following the trend. They're jumping on — saying, you know, 'I don't care about valuation, I'm going for this ride.' And that worries me a little bit," said Siegel.
As of Thursday's close the Dow was just 2.4% away from 30,000. The year may be just two weeks old, but the index has already seen six all-time highs.
"You can't keep on expanding multiples. You have to have earnings follow in a meaningful way," Siegel said. "At this point, we have not seen earnings following a meaningful way."
Siegel, who accurately predicted market milestones including Dow 20,000 and Dow 25,000, speculates the market is becoming more and more vulnerable to a 10% sell-off.
"Any little thing could trip things up. You know, earnings disappointments," he said. "The market is going to be vulnerable for whatever disappointments, whatever bump that happens. Is Iran completely over? Is it solved? Do we have nothing to worry about in Europe or anywhere else internationally?"
Plus, Siegel warns that November's presidential election is emerging as a serious risk factor.
"There's a lot of uncertainty," he said. "If there's a Democratic sweep, both of the presidency and the Senate, I think that would be very bearish for the market because the Democrats say they're going to undo the corporate tax cuts that [President Donald] Trump put in."
Even though it's a bearish sounding forecast, Siegel isn't sounding the alarm on the bull market. His immediate warning applies particularly to short-term investors.
"Long-term investors, I think, are fine. There is no reason to panic and start moving out," Siegel said. "But [to] those people who might be thinking of taking some profits over the next few weeks or so, watch this market closely."