What happened

Shares of J.C. Penney (NYSE:JCP) slumped on Thursday, one day before the retailer is set to report its fourth-quarter results. There was no news driving the decline, but a broad market sell-off may be partly to blame. The stock was down about 11% at 3:20 p.m. EST.

So what

J.C. Penney will report its full fourth-quarter results before the market opens on Friday. The company has already reported holiday sales numbers, delivering a 3.4% increase in comparable sales for the nine-week period ending Dec. 30 It reaffirmed its full-year guidance along with that announcement.

A J.C. Penney storefront.

Image source: J.C. Penney.

Both Macy's (NYSE:M) and Kohl's (NYSE:KSS) have already reported mostly positive results. Macy's reported a 1.4% increase in comparable sales, and it managed to boost earnings even after backing out significant asset sales during the quarter. Kohl's reported 6.3% comparable sales growth, and it managed to grow earnings as well.

J.C. Penney's results will feature comparable sales growth, and its bottom line is likely to improve. But the company is still struggling to turn much of a profit on a depressed sales base. It has managed an operating margin of just 1.1% over the past 12 months, although a strong fourth quarter could boost that figure.

Now what

Pre-earnings jitters, combined with a market sell-off, may be what's pushing down shares of J.C. Penney. The stock has crashed 80% from its five-year high, with the market becoming increasingly pessimistic over the past year about the company's turnaround prospects. Investors will know more on Friday morning when J.C. Penney announces its full results.