France rejects a U.S. proposal this week that would let companies opt out of a proposed international tax reform, Finance Minister Bruno Le Maire said on Friday, urging Washington to negotiate in good faith.

U.S. Treasury Secretary Steven Mnuchin raised serious questions about OECD international tax reform proposals in a letter made public on Wednesday, jarring international officials by floating the idea of a “safe harbor regime”.

Le Maire said that would mean U.S. companies could opt in or out as they pleased, which he said would be unacceptable to France and other OECD countries.

He urged Washington therefore to negotiate on the basis that the new tax rules be binding, and said if the efforts at the OECD fell through EU countries should revive talks for a European digital tax.