Canada became the second country in the world to legalize marijuana, with stores opening on Wednesday. But the United States, with its own strict laws still in place at the federal level, Canadians involved in the business — or even smoking a joint — may well find trouble at the U.S border. The United States should quickly resolve this issue.

Even though nine U.S. states allow recreational marijuana use, since it remains illegal at the federal level, U.S. Customs and Border Protection agents are able to exercise broad discretion in turning away Canadians for a range of marijuana-related offenses.

For example, an agent at the border might ask a Canadian entering the U.S. if he smoked cannabis. If he said yes, then that alone could be grounds to turn him away.

Not only did that individual not break any law, but was participating is a legal industry selected by a democratic majority in a close ally and neighbor. The impact of denying such entries either prevents people from exercising their rights in Canada if they must come to the U.S. or pushes Canadians to look for more friendly places to spend their money.

That would be bad for the U.S. economy, bad for border states, and bad for bilateral relations.

This isn’t to say that Canadians should be able to bring drugs into the country or seek to do business in marijuana here. But the mere crossing of the border doesn’t require asking about legal personal use.

With the first day of legalization almost in the books, the agitation around a potential border showdown looks to be a nonissue.

According to an Associated Press report, the new law in Canada hasn’t had an impact on the flow of traffic at the U.S. border. In Detroit, reporters were told by Christopher Perry of CBP that questions about smoking pot won't be routine and will only be asked if officers have reason to be suspicious.

That policy of not making it an issue might work for now, but it should be replaced with a comprehensive provision that would prevent Canadians from facing difficulty at the border over cannabis use and participation in a legal industry in their own country. Congress would be wise, especially to prevent law-abiding Canadians crossing the border from being used as potential bargaining chips in President Trump’s next dust up with Prime Minister Justin Trudeau.