The NFL is asking prediction market operators to keep specific event contracts that the league deems “objectionable bets” off of their platforms.

In a letter obtained by CNBC, the league outlines examples of event contracts that would be easily manipulable by a single person, inherently objectionable, related to officiating and knowable in advance — and asks that operators refrain from offering such trades.

The NFL declined to comment on which companies received the letter, but said it was sent to operators that are registered with the Commodity Futures Trading Commission and that offer NFL trades.

Prediction platforms Kalshi and Polymarket have dominated the burgeoning predictions industry in recent months, spurring sports betting incumbents like FanDuel and DraftKings to enter the predictions space, as well.

“Sports prediction markets are not effectively regulated currently,” NFL executive vice president Jeff Miller said in a statement. “We will continue to engage with the CFTC in pursuit of the necessary guardrails to protect both the integrity of the game and consumers participating in these rapidly evolving markets.”

While some leagues such as the NHL, MLB and MLS have embraced prediction markets, signing operators as partners, the NFL has been more cautious.

“There is no greater priority for the NFL than protecting the integrity of our games and the welfare of our players,” the letter stated.

In the letter, signed by NFL Chief Compliance Officer Sabrina Perel, she says it is encouraging that the CTFC recognizes that sports-related prediction markets should be regulated differently than other futures contracts.

The examples provided in the letter of events that could be easily manipulated by a single person included whether a kicker would miss a field goal, a quarterback’s first pass being incomplete, or a receiver missing their first target.

The list also included non-game related event contracts, such as broadcast mentions, or appearances by fans or celebrities at the games. During the Super Bowl, these types of wagers were extremely popular, such as whether Jeff Bezos would be in attendance.

Kalshi CEO Tarek Mansour told CNBC after the February championship game that the prediction platform saw more than $100 million in trading volume alone on a question of what halftime performer Bad Bunny’s first song would be.

The league also took issue with “inherently objectionable” wagers such as play injuries, fan safety and play misconduct.

The letter concludes by saying the NFL would be happy to meet to discuss “our views on sports prediction markets in greater detail, including prohibited bettors, information sharing with leagues and responsible betting measures.”

Disclosure: CNBC and Kalshi have a commercial relationship that includes a CNBC minority investment.