HELENA — A bill meant to combat “revenge porn” seemed on its way to passage in the Montana Legislature, but it took an unexpected turn Friday when the state Senate unanimously rejected the measure.

The measure sought to make it illegal to distribute sexual images of a person without that person’s consent. About three dozen states have laws against nonconsensual porn, which has become a national concern because of the ubiquity of cellphone cameras and the speed in which unauthorized images can spread on social media and other platforms.

The Senate had given the measure its initial blessing Wednesday on a 39-11 vote. But two days later, the chamber killed it with an unusual 0-50 vote during what was supposed to be a final and routine rubber-stamping vote.

The bill’s doom was met with murmurs on the Senate floor after the dramatic reversal.

While bills sometimes get rejected on third reading, those rare occurrences usually befall contentious bills garnering tighter margins during earlier votes.

Pressure from the Montana Coalition Against Domestic and Sexual Violence partly prompted the reversal.

“The strongest statute is one that places the focus of the crime on a perpetrator who chooses to distribute the images without the consent of the victim for any reason whatsoever. The focus of an effective statute is properly on the perpetrator, not on the victim,” the group said in an email from Robin Turner, its public policy and legal director.

Turner declined an interview request.

The bill hardly seemed controversial when it passed the House 95-5 last month before being sent to the Senate.

The bill’s main sponsor, Democratic Rep. Ellie Hill Smith of Missoula, blamed a “poison pill” that was dropped into the bill as it wound through the Senate. But she and others couldn’t fully explain what occurred.

“I’m just learning about this today,” she said. “This is all unfortunate because this bill was the result of 15 months of bipartisan work to update our sexual assault laws.”

Sen. Tom Facey of Missoula, a top member of the Democratic Senate leadership, said there was clearly some confusion.

“There was some miscommunication between some of the stakeholders and some our Senators. The miscommunication had to do whether the amendments were acceptable to the stakeholder groups,” Facey said.

While the Senate Judiciary Committee had tweaked some of the language before voting 8-3 to send it to the Senate floor, a seemingly small amendment during a Senate floor hearing helped push the bill to its doom.

During its second hearing on Wednesday, Republican Sen. Keith Regier won overwhelming support to insert the phrase “financially profit” into a section of the bill.

“The bill was barely acceptable before that amendment got on,” Facey said. “It went from barely acceptable to finally saying that it was not a good deal at all.”

But it remained unclear, even to Facey and other Senators from both sides of the aisle, what specific language caused the anti-sexual violence coalition to object to the final version of the bill.