Shares of O’Reilly Automotive, parent of the O’Reilly Auto Parts stores, dropped about 20% Wednesday after the company announced smaller sales growth than expected.

The auto parts retailer, which operates 4,888 stores in 47 states, expected 3% to 5% growth in the second quarter of 2017. But in a statement released Wednesday, O’Reilly reported same-store sales growth of just 1.7% over the period. Same-store sales growth is an industry measure that excludes results from newer stores.

O’Reilly shares fell 18.9% to close Wednesday at $178.77, down $41.64.

Greg Henslee, O’Reilly’s CEO, blamed two consecutive mild winters and “overall weak consumer demand” for the lower-than-expected growth.

““While we are disappointed with our sales results in the first half of the year, we remain confident in the long-term health of our industry and our team’s ability to provide exceptional customer service and take market share in this challenging demand environment,” Henslee said in a statement.

But Efraim Levy, an equity analyst with CFRA Research, said that competition with online retailer Amazon might be hurting brick-and-mortar stores like O’Reilly.

“While (O’Reilly) blamed the mild winter and weak demand, we think this exacerbates concerns about encroachment from, which is dragging down other parts retailers too,” Levy said in a note.

Advance Auto Parts fell 11.2%, AutoZone sank 9.6% and Genuine Parts Company, which owns Napa Auto Parts, dropped 4.8%.

Before Wednesday’s plunge, O’Reilly’s stock had already dropped 16% over the last quarter. The latest fall made it the worst performer in the S&P 500 index for the day.

O’Reilly will report second-quarter earnings on July 26.