While many Americans may not be familiar with the wealthiest man in Japan, billionaire Tadashi Yanai, they likely know the colorful clothing brand Uniqlo, which has aggressively expanded into the US in its quest to take on H&M and other fast-fashion chains.

Yanai is the CEO of Fast Retailing, which owns Uniqlo (along with J Brand, Theory, and others). And on Friday, Yanai lost an eye-popping $1.4 billion in paper wealth.

The reason? Uniqlo reported that its same-store sales declined 5% in December. That report led shares of Fast Retailing (FRCOY) to drop nearly 7% in one day. Uniqlo makes up about 80% of Fast Retailing’s revenue, and Yanai’s wealth comes from his stock holdings in Fast Retailing.

As a result of Fast Retailing’s stock drop, Yanai slipped six slots on the Bloomberg Billionaires Index to No. 44 in the world, with an estimated $17.7 billion. It is the biggest fall for any billionaire on the list so far in 2017.

View photos Fast Retailing stock chart in the past week More

So, why did Uniqlo’s sales decline in December?

It was the weather, the company said. “Same-store sales declined year on year in December after warm weather in the latter part of the month dampened sales of core winter items,” Uniqlo said in its earnings report.

That may sound like a ridiculous excuse, but it’s not such a stretch. Uniqlo has been pushing its HeatTech products with particular emphasis, putting much of its marketing budget into promoting the fabric, which has “heat retaining properties to keep you warm and a soft texture to keep you comfortable all day,” Uniqlo’s marketing language claims. But unseasonably global high temperatures hit many fashion retailers hard this holiday season.

Yanai may have to make the company less dependent on seasonal trends if he wants to achieve his stated goal of bringing Uniqlo to $10 billion in US sales by 2020—and if he wants to claw back some of that personal wealth.

Uniqlo’s unique approach to sports and culture marketing, involving regional “local ambassadors” as well as a few big-name “global ambassadors” like tennis star Novak Djokovic, may help.

It also recently announced an unusual partnership with FamilyMart Co., Ltd. and Lawson, Inc. to launch a convenience store pick up service. The service will enable customers in Japan to order clothing online and pick it up at a local 7-Eleven, FamilyMart or Lawson store, 24 hours per day. It’s the kind of innovation necessary if Uniqlo wants to bounce back.

Additional reporting and writing in this story by Daniel Roberts.

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Jame Jackson is a social writer at Yahoo.

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