Permanent stock market bull Jeremy Siegel doesn't seem so bullish about economic growth last quarter. And if growth doesn't pick up, it could derail one of the most bullish stock market calls for this year.

Siegel, a finance professor at the University of Pennsylvania's Wharton School, told CNBC on Friday that he thinks last quarter's GDP numbers could come in as low as 1 percent. That's too low to support his forecasts for the Dow surpassing 18,000 points.