One worker at the Hudson Yards megaproject is being treated for severe but non-life-threatening injuries after construction scaffolding collapsed.

Three other workers also sustained minor injuries,, the New York Post reports. The incident was at 50 Hudson Yards, and it is not yet clear what caused the problem.

Two of the workers fell from the 18th floor to the floor beneath, the New York Daily News reports. All four of the people were taken to the hospital.

This is the first time there has been an incident at the construction site, which was the subject of a controversial deal struck last year by developer Related and a powerful union group. Related had previously sued the Building and Construction Trades Council, the largest construction union group in the city, alleging it was improperly holding up construction at 50 Hudson Yards.

The under-construction building is being developed by Related and Oxford Properties Group and is set to span 2.9M SF and cost around $3.8B.

BlackRock signed on to take more than 800K SF there, and rumors have been swirling about other potential leases in the works — including Facebook and law firm Debevoise & Plimpton.

New York construction deaths spiked in recent years leading to a 2017 bill that passed the City Council that requires workers on most construction sites to receive a minimum of 40 hours of safety training.