DIVNA MASLENJAK may be an unsympathetic standard-bearer for an important legal principle. An ethnic Serb who arrived in the United States as a refugee from Bosnia, she became a U.S. citizen a decade ago after misleading an immigration judge about her husband’s military record during the Balkan wars. She said he managed to escape military conscription; in fact, he served in a unit that was implicated in war crimes.

Ms. Maslenjak was wrong to tell an apparent lie, and a jury might well look kindly on the government’s contention that she had received her citizenship illegally. But it’s a stretch to think a jury would regard any lie — relevant or irrelevant, odious or trivial — with the same degree of disapproval.

What if, Chief Justice John G. Roberts Jr. asked the government’s lawyer when Ms. Maslenjak’s case was heard at the Supreme Court the other day, he drove 60 miles an hour in a 55-mile-an-hour zone, then, having not been arrested, failed to mention it on a naturalization application. “If I answer the question ‘no,’ 20 years after I was naturalized as a citizen, you can knock on my door and say, ‘Guess what, you’re not an American citizen after all’?” he asked.

The Justice Department lawyer, Robert A. Parker, never fully recovered from the chief justice’s line of inquiry, although he labored valiantly to make a case for legal absolutism — that any lie is as bad as another. As several other justices pointed out, however, his stance was an affront to common sense, and to previous court rulings suggesting that an irrelevant falsehood, one likely immaterial to a naturalization case’s outcome, could not reasonably be grounds for subsequently revoking citizenship.

Mr. Roberts, always alert to the potential for prosecutorial overreach, was right to raise the specter of abuse. Justice Sonia Sotomayor piled on, asking if the government would also rescind citizenship from a naturalized immigrant who had failed to divulge an insulting childhood nickname, as the form requires. Given sufficient scrutiny, there may be relatively few refugees or applicants for naturalization whose sworn statements turn out to be gospel. That goes for the rest of us, too, who may wish to avoid divulging items from our past that are embarrassing, painful or illegal — including speeding violations as in Mr. Roberts’s hypothetical.

The government was on shaky ground by arguing that the crush of annual petitions for naturalizations — about 800,000 of them are filed annually — makes it unlikely that prosecutors could muster the resources to comb through them all for evidence of dishonesty. The point is not that naturalized immigrants would be stripped of citizenship en masse; it is that nearly anyone might be singled out if even the most inconsequential lie were grounds for prosecution and revocation of citizenship.

Justice Anthony M. Kennedy rightly took umbrage at the government’s indiscriminate formulation, pointedly telling Mr. Parker that “your argument is demeaning the priceless value of citizenship.” The court seems on solid ground in insisting that citizenship, once granted, should not be taken back blithely.