The by-election was called after the Constitutional Council invalidated the election of Samantha Cazebonne, candidate of La République En Marche!

Sailliet claimed that Cazebonne was present at a meeting of business leaders over a meal at a restaurant in Barcelona on 13 June that was financed by the France-Spain Chamber of Commerce and Industry.

However, the records Cazebonne submitted to the National Commission for Campaign Accounts and Political Financing (CNCCFP) showed that the meal was paid for by each of its participants, not the France-Spain Chamber of Commerce and Industry, and this complaint was therefore dismissed.

Ralle-Andreoli, who emailed a message at 20:03 local time on 3 June 2017 to some voters registered on electoral rolls within the constituency, violated this article.

Given these issues and the small margin separating the second- and third-placed candidates in the first round, the constitutional council annulled the result of the election,[4][5] triggering a by-election within three months.