The Supreme Court on Dec. 15 said a police officer made a “reasonable” mistake in misunderstanding a law about traffic stops. (Andrew Harrer/Bloomberg)

The Supreme Court was forgiving of a police officer’s misunderstanding of the law Monday, ruling that it did not invalidate a traffic stop that led to a drug bust.

The justices ruled 8 to 1 that a North Carolina officer made a reasonable mistake when he pulled over a car with a broken brake light, which led to the arrest of Nicholas Brady Heien for attempted trafficking in cocaine.

It turns out that North Carolina’s antiquated law requires only one functioning “stop lamp,” not two. But that doesn’t mean that the search of Heien’s vehicle violated the Constitution’s protection against unreasonable searches and seizures, Chief Justice John G. Roberts Jr. wrote for the majority .

“To be reasonable is not to be perfect, and so the Fourth Amendment allows for some mistakes on the part of government officials, giving them fair leeway for enforcing the law in the community’s protection,” Roberts wrote.

He said the court previously has ruled that searches based on mistaken facts are not automatically unlawful, the limit being that the “mistakes must be those of reasonable men.”

Roberts added: “But reasonable men make mistakes of law, too, and such mistakes are no less compatible with the concept of reasonable suspicion.”

Justice Sonia Sotomayor was the lone dissenter. The decision “means further eroding the Fourth Amendment’s protection of civil liberties in a context where that protection has already been worn down,” she wrote.

Heien was in the back seat, a friend driving, when they caught the attention of Surry County Sheriff’s Sgt. Matt Darisse in April 2009. Darisse thought the driver, Maynor Javier Vasquez, looked “very stiff and nervous,” so the officer followed for a bit. When he noticed the right brake light was not working, he pulled the car over.

He was writing a warning when he became more concerned. The two gave inconsistent answers about where they were going and acted suspiciously, according to Darisse. The officer asked if he could search the car, and eventually Heien, who owned the vehicle, agreed. In the side compartment of a duffle bag, Darisse found a baggie containing cocaine.

Heien’s conviction was thrown out by a North Carolina appeals court. Since the state’s law did not require two working rear lights, the court reasoned, Darisse’s justification for the stop was “objectively unreasonable.”

The North Carolina Supreme Court reversed that decision. Other courts around the country are divided on the questions that arise when an officer makes a mistake of law.

Heien and those who supported his case pointed to what Roberts called “the well-known maxim” that ignorance of the law is no excuse.

They “contend that it is fundamentally unfair to let police officers get away with mistakes of law when the citizenry is accorded no such leeway,” Roberts wrote.

But he said the argument had only a “certain rhetorical appeal.”

“Just because mistakes of law cannot justify either the imposition or the avoidance of criminal liability, it does not follow that they cannot justify an investigatory stop,” Roberts wrote. “And Heien is not appealing a brake-light ticket; he is appealing a cocaine-trafficking conviction as to which there is no asserted mistake of fact or law.”

Roberts stressed that the exception applies only to reasonable mistakes and said “an officer can gain no Fourth Amendment advantage through a sloppy study of the laws he is duty-bound to enforce.”

Justice Elena Kagan, joined by Justice Ruth Bader Ginsburg, filed a concurrence to emphasize that point. “That means the government cannot defend an officer’s mistaken legal interpretation on the ground that the officer was unaware of or untrained in the law,” she wrote.

The case is Heien v. North Carolina.