President-elect Donald Trump may need to start thinking of new ways to fund his big, expensive, irrational wall.

Trump has repeatedly promised that Mexico will pay for a wall on its border with the U.S. in order to keep undocumented immigrants out. On Wednesday, Trump clarified that the United States would begin building the wall immediately and force its southern neighbor to “reimburse us.” But Mexican President Enrique Peña Nieto insisted that’s not going to happen, calling the wall “against our dignity.”

“It is evident that we have differences with the new United States government on some issues, such as a wall that Mexico absolutely will not pay for,” Peña Nieto said on Wednesday, according to the Guardian. “At no time will we accept anything that goes against our dignity as a country and our dignity as Mexicans.... basic principles such as sovereignty, the national interest and the protection of our citizens are non-negotiable.”

This isn’t the first time Peña Nieto has expressed opposition to the wall, which experts have estimated would cost around $25 billion.

“I made it clear that Mexico will not pay for the wall,” Peña Nieto said in August, after meeting with Trump.

His predecessors have also lashed out at Trump’s plan.

“I’m not going to pay for that fucking wall,” former Mexican President Vicente Fox told Fusion last year.

“Mexican people, we are not going to pay any single cent for such a stupid wall, and they need to know that,” former Mexican President Felipe Calderón told CNBC.