Apple’s shares were halted after the stock market closed Wednesday. Then CEO Tim Cook posted a letter on Apple’s web site saying that revenue in the last quarter of 2018 would be $84 billion.

That figure is 4.8% below the $88.3 billion revenue in the same quarter a year earlier. It’s also far short of the forecast Apple gave investors only two months ago, which saw revenue in the quarter coming in between $89 billion and $93 billion.

“We expected economic weakness in some emerging markets. This turned out to have a significantly greater impact than we had projected,” Cook said in the letter. “In addition, these and other factors resulted in fewer iPhone upgrades than we had anticipated. These last two points have led us to reduce our revenue guidance.”

While Cook remained optimistic that Apple has a “bright future” in China, flagging sales in the country are a key reason for its lower revenue outlook.

“While we anticipated some challenges in key emerging markets, we did not foresee the magnitude of the economic deceleration, particularly in Greater China. In fact, most of our revenue shortfall to our guidance, and over 100 percent of our year-over-year worldwide revenue decline, occurred in Greater China across iPhone, Mac and iPad,” Cook wrote.

The disappointing sales was strongest for iPhones, which has accounted for about three fifths of the company’s revenue. “Lower than anticipated iPhone revenue, primarily in Greater China, accounts for all of our revenue shortfall to our guidance and for much more than our entire year-over-year revenue decline,” Cook said, noting that iPhone upgrades in developed markets like North America and Europe “also were not as strong as we thought they would be.”

Apple’s stock closed official trading up 0.1% at $152.92 a share. Once trading on the stock resumed in after-hours trading, the stock fell 8% to $145.15 a share.