Kylie Jenner's make-up company has warned customers of a security breach compromising names, addresses and the last four digits of credit cards.
Shopify said two "rogue" support-team workers had stolen customer data from at least 100 sellers on its platform.
Kylie Cosmetics said it was among those affected but was confident customers could continue to buy on its Shopify-run website.
"Your trust is so important to us," the company told them in an email.
"And we wanted to let you know we're working diligently with Shopify to get additional information about this incident and their investigation and response to this matter.
"Shopify has assured us that they have implemented additional controls designed to help prevent this type of incident from recurring in the future."
Last year, billionaire Kylie Jenner - who rose to fame on TV series Keeping Up with the Kardashians - sold most of her shares in the company, for $600m (£463m).
Beauty giant Coty now controls 51% of the 23-year-old's cosmetics brand.
More than one million merchants use Shopify, including Taylor Swift, Victoria Beckham and Tesla.