Seth Dallaire, Instacart’s Chief Revenue Officer, has left the grocery delivery startup to take on the CRO role at Walmart, per a report from Business Insider.

Dallaire is one of the first major defections from Instacart after it recruited a host of c-suite talent earlier this year ahead of a widely expected IPO later this year.

While at Instacart, he had been a core figure in building up the company as a destination for consumer-goods companies to advertise on Instacart’s platform. Before that, he had a similar role at Amazon as the e-commerce behemoth's VP of Global Ad Sales.

His departure is a major setback for Instacart, which hopes to generate between $10 billion and $20 billion in annual revenue from ads in the future. However, Instacart isn’t completely rudderless as it recently onboarded Facebook ad guru Carolyn Everson as its new President. While at Facebook, Everson had been the main architect of and point of contact for the social media titan’s relationship with marketing and ad executives.

At Walmart, Dallaire will help craft an advertising business based off of Walmart’s considerable and growing e-commerce business — online sales at the retail giant are up 103% over the past two years. If successful, Dallaire could help grow Walmart's ad business into a significant revenue source, as rival Amazon now generates more revenue from ads than Snap, Twitter, Roku, and Pinterest combined.