President Donald Trump said Canada would need to pay $61 billion to join the “Golden Dome” system he’s proposed to boost missile defense with space-based interceptors.

But the president needled Ottawa by saying he would be willing to protect Canada for free if the nation opted to join the United States, his latest suggestion that the northern neighbor would benefit from being absorbed by the U.S.

“I told Canada, which very much wants to be part of our fabulous Golden Dome System, that it will cost $61 Billion Dollars if they remain a separate, but unequal, Nation, but will cost ZERO DOLLARS if they become our cherished 51st State,” Trump said in a social media post on Tuesday. “They are considering the offer!” Trump last week detailed his ambitious effort to build a shield to protect the U.S. from threats including ballistic missiles, hypersonics and advanced cruise missiles, saying that it would be fully operational by the end of his term. The technology for Trump’s plan is unproven, particularly using space-based interceptors to knock down incoming ballistic missiles.

— The Associated Press