President-elect Donald Trump’s remarks that Canada should become the 51st state drew condemnation and a patriotic outburst on Saturday from a former Canadian prime minister who offered blunt advice to the incoming U.S. leader: “Give your head a shake!”

Jean Chrétien, who was Canada’s prime minister from 1993 to 2003, joined a chorus of officials from the northern U.S. neighbor who say Trump’s remarks are no longer a joke and may undermine America’s closest ally.

Canada would never agree to become part of the United States, Chrétien wrote in an article published in The Globe and Mail newspaper, celebrating his 91st birthday.

He extolled his nation’s love of independence and said Trump’s remarks amounted to “totally unacceptable insults and unprecedented threats” to Canadian sovereignty.

“To Donald Trump, from one old guy to another, give your head a shake!” Chrétien said. “What could make you think that Canadians would ever give up the best country in the world — and make no mistake that is what we are — to join the United States?

Trump has tossed expansionist rhetoric not just at Canada but also at other U.S. allies, with arguments that the frontiers of American power need to be extended to the Danish territory of Greenland, and southward to include the Panama Canal.

And while many European leaders have been measured in their response, Canadians have not held back.

“If you think that threatening and insulting us is going to win us over, you really don’t know a thing about us,” Chrétien wrote in the article. “We may look easy-going, mild-mannered. But make no mistake, we have spine and toughness.”

The U.S. imports approximately 60% of its crude oil from Canada, which is also the top export destination for 36 U.S. states. Nearly $3.6 billion Canadian ($2.7 billion) worth of goods and services cross the border each day.

Canadian officials have been talking to incoming Trump administration officials about increasing border security in an effort to avoid a sweeping 25% tariff that Trump has threatened to impose on all Canadian products.

When Trump imposed higher tariffs during his first term in office, other countries responded with retaliatory tariffs of their own. Canada announced billions of new duties in 2018 against the U.S. in a tit-for-tat response to new taxes on Canadian steel and aluminum.

Canada former Prime Minister Chrétien advises Trump to ‘Give your head a shake!’ as he rejects 51st state remarks