Chris Hemsworth, the actor behind Thor, has joined the fight against the construction of the Dakota Access Pipeline with an Instagram post that also apologizes for dressing as a Native American at a New Year’s Eve theme party last year.

The 33-year-old wrote on Instagram that he supported the Standing Rock Sioux tribe and demonstrators who want to stop the production of a 1,200-mile Dakota Access oil pipeline that would carry crude oil from North Dakota’s Bakken oil fields to Illinois. “Standing with those who are fighting to protect their sacred land and water. #nodapl #waterislife #mniwiconi,” he wrote, dressed as Thor, pinging director Taika Waititi, who is behind Thor: Ragnarok.

In the second half of his post, he apologizes to all First Nations people for dressing up as a Native American, saying it was a “thoughtless action.” “Last New Year's Eve I was at a ‘Lone Ranger’ themed party where some of us, myself included, wore the traditional dress of First Nations people,” he says. “I was stupidly unaware of the offence this may have caused and the sensitivity around this issue. I sincerely and unreservedly apologise to all First Nations people for this thoughtless action.”

He added: “I now appreciate that there is a great need for a deeper understanding of the complex and extensive issues facing indigenous communities. I hope that in highlighting my own ignorance I can help in some small way.”

Shailene Woodley, who was one of the first celebrities to speak up and recently broke her silence following her arrest, thanked Hemsworth on Twitter.

According to Entertainment Weekly, Hemsworth joins Mark Ruffalo, Susan Sarandon, Nikki Reed, Leonardo DiCaprio, and Jaden and Willow Smith, who have all pledged their allegiance in standing with Standing Rock.