Jeremy Siegel is one of the most famous market bulls, and he reiterated his positive take on Tuesday with a call for the Dow Jones Industrial Average to rise to 18,000. But he also mentioned two things that worry him right now: a potential lack of slack in the labor force, and commodity prices.

(Read more: We're only in the bull market's 4th inning: Siegel)

Siegel, a Wharton professor of finance, said on Tuesday's episode of "Futures Now" that a sharp rise in commodity costs could change his take entirely.

"If I saw commodities really increasing in price—I mean, we've had a little bump in oil, we know why we had that yesterday, with the threat of war, and shutoffs of supply, and embargoes against Russia—but anything that sparks any inflation at all" is a serious concern, Siegel said.

(Read more: Ukraine tensions boost oil but resistance is strong)

"The commodity prices have been holding up better than I would think given the slowness in the economy. But if that started flaring up, not only is taper going to be maintained—it could be accelerated by the Fed," Siegel said.

The issue is that the Federal Reserve's quantitative easing program, and its ultralow federal funds rate, both depend on low inflation. If rising energy prices suddenly spur inflation, then the Fed could be forced to pull back, potentially causing interest rates to spike and thus hurting the market.

"If we see some of these commodity prices rise up the way we did yesterday to continue to rise up," then the Fed is "going to be geared" to cut back on quantitative easing and increase the federal funds rate.

For that reason, rising commodity prices "would be a concern for me," Siegel said.

Siegel's other concern, which also pertains to rising costs, is the labor force.

While many complain that the unemployment rate looks artificially low due to the unusually low participation rate, Siegel frets that it could actually be lower than the stated 6.6 percent.

That might sound like good news, but it would mean that if companies need to hire additional workers, then they need to raises wages. That would put a serious damper on the corporate profits that undergird stock prices.

(Read more: What the minimum wage fight says about the economy)

"The labor market—even though the unemployment rate is 6.7 percent, 6.6 percent—it might be tighter than we think," Siegel said. "We might be closer to a bottleneck there—although we are seeing no wage increases—than we are concerned [about]. And that does concern me. Because these optimistic earnings estimates won't be reached unless we still have slack in the labor market to employ people."

These concerns aside, Siegel still recommends that investors buy stocks, saying we could be in "about the fourth or fifth inning of the bull market."

—By CNBC's Alex Rosenberg. Follow him on Twitter: @CNBCAlex.

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