At a meeting with investors in New York, Walmart announced that its sales are expected to be flat this year. The retailer previously forecast that net sales would grow 1 to 2 percent for the current fiscal year, through January.

The company explained that considerable investments in wages and in e-commerce would affect future earnings and slashed its sales forecast for the year. Walmart’s chief executive Doug McMillon says that the company needs to invest to return to growth.

The news startled investors. Walmart shares plunged after the announcement, leading to their worst one-day slump in 17 years. Shares in Walmart fell 10.04 percent to reach $60.03. The drop resulted in the loss of more than $20 billion in market value. So far this year, Walmart’s shares are down 30 percent.

Walmart has dominated the world of retail for decades. Walmart’s superstores once upended the way people shopped, but now it appears that its performance is slowing. Walmart has been facing increased competition in the e-commerce market. Amazon eclipsed Walmart as the world’s biggest retailer by market capitalization this year.

The company is investing in a number of improvements, including investing significant capital into its online and delivery operations. Walmart also announced a new $20 billion share buyback program.

Investors are also worried about expected weak holiday spending this year. The National Retail Federation estimated that sales for November and December will rise 3.7 percent to $630 billion, lower than the 4.1 percent gain last year. The Commerce Department released figures earlier that showed overall retail sales for September were virtually flat.