The judicial opinions of Supreme Court Justice Antonin Scalia can be divisive. His writing’s substance and style often thrill the right and enrage the left. Yet his vigorous dissent from a recent sentencing decision, Sykes v. U.S., is enjoying a warm reception from conservatives and progressives — and rightly so.

In his dissent, Scalia attacked the “tutti-frutti opinion” of the majority, which interpreted an extremely vague provision of a federal law to mean that state law against using a car to flee the police was a “violent felony” and, as a result, the convicted man was subject to a much longer prison sentence.

What bothered Scalia was not the harsh punishment but rather the attempt by the court — the fourth in five years — to interpret this provision of the federal criminal code, known as the “residual clause” of the Armed Career Criminal Act. According to Scalia, the court should stop trying to read into the law what Congress failed to specify and instead should hold the residual clause unconstitutional for vagueness.

Scalia is right. Due process requires that the criminal law provide fair notice of what activities are prohibited and which acts will result in a sentence enhancement, especially for crimes that are not clearly immoral on their face. It is fundamentally unfair to deprive someone of liberty or property for an act that he or she had no reason to know was criminal. Although it’s often said that “ignorance of the law is no defense,” that well-worn phrase does not apply when a law is so vague that its provisions are indeterminable.

Moreover, vagueness can render criminal law ineffective. Deterrence — a basic purpose of sentencing enhancements such as those in the Armed Career Criminal Act — depends entirely on the ability of a reasonable person to distinguish and therefore to avoid prohibited acts.

Fortunately, the court has not always upheld unclear criminal laws. Last year, it rightly recognized in Skilling v. U.S. that the “honest services fraud” statute was too vague to satisfy constitutional requirements of fair notice.

Unfortunately, Congress hasn’t gotten the message. Two recently introduced bills — the Honest Services Restoration Act (H.R. 1468) and the Clean Up Government Act (H.R. 1793) — contain language so vague that it is hard for anyone to tell what they would criminalize.

For example, the Honest Services Restoration Act would criminalize “undisclosed self-dealing” by any public official. It sounds pretty sinister, but what exactly does “undisclosed self-dealing” mean?

As written, the bill might criminalize a vote by a state legislator in favor of financial aid for higher education if that person has a toddler who might use that financial aid program to someday pay for college. Or it might not. We just don’t know because the text is vague and unclear.

These bills are so vague that, if they are enacted, it would be the responsibility of prosecutors and judges to interpret what they criminalize, and the public won’t know what that is until someone is convicted of it. Like other unclear criminal laws, these bills would give enormous power to prosecutors, whose charging authority makes them the initial decision-makers about whether a person’s actions fall within the prohibited actions under a vague law.

I don’t question the good intentions behind these bills — nobody likes government corruption. But when Congress decides to create a new crime, it should do so with care and specificity sufficient to ensure fair notice for the public.

Scalia’s Sykes v. U.S. dissent noted this problem of imprecise, hurried drafting of criminal statutes: “Fuzzy, leave-the-details-to-be-sorted-out-by-the-courts legislation is attractive to the Congressman who wants credit for addressing a national problem but does not have the time (or perhaps the votes) to grapple with the nitty-gritty. In the field of criminal law, at least, it is time to call a halt.”

Congress should stop delegating the determination of what should be a crime to prosecutors and judges, who can only act after an alleged violation. If the purpose of the law is to prevent disfavored behavior, then relying on the court system to figure it out after the fact is a clear failure of deterrence. If the accused was acting in good faith, as is often the case with charges brought under a vague criminal statute, there is also a clear failure to provide fair notice.

It is time for Members of Congress to stop creating criminal provisions that are so vague not even the justices of the Supreme Court can fully determine what’s been criminalized. If the brilliant judges on the highest court in the land cannot understand what a law says, how can the average American expect to know what that law prohibits?

Virginia Sloan is president of the Constitution Project.