Sia Explains Casting Maddie Ziegler as Character on Autism Spectrum: 'Can't Do a Project Without Her'

Sia is further explaining why she chose to cast frequent collaborator Maddie Ziegler as a character on the autism spectrum in her new movie.

The 45-year-old "Chandelier" singer will make her directorial debut with the film Music, set to be released in early 2021. When the first trailer debuted in November, many social media users expressed outrage, questioning why Sia cast the Dance Moms alum, 18, to play the titular role rather than an actor on the spectrum.

The movie, which also stars Kate Hudson as Zu, follows a recently sober drug dealer who suddenly becomes the guardian of her younger sister named Music, a special-needs teen who communicates through a device that speaks for her, and who always listens to music via large headphones.

In an interview with Australian talk show The Project over the weekend, Sia said she couldn't do the movie without Ziegler. The young dancer has appeared in countless Sia music videos and visuals over the years.

"I realized it wasn't ableism — I mean, it is ableism, I guess, as well — but it's actually nepotism, because I can't do a project without her, I don't want to. I wouldn't make art if it didn't include her," said Sia.

Sia added that Ziegler was concerned about portraying the role at first, but the first-time director assured that she would protect her from backlash — something she's found out she isn't able to do.

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"She cried on the first day of rehearsals and she was really scared. She just said, 'I don't want anyone to think I'm making fun of them,' " Sia recalled. "And I, bald-facedly, said 'I won't let that happen.' And last week I realized I couldn't really protect her from that, which I thought I could."

She continued, "... I've realized there are some things I can't protect her from, as much as I try. I guess that's like any mom or bonus mom would say."

A rep for Ziegler did not immediately respond to PEOPLE's request for comment.

After the first trailer for the movie dropped in November, critics and autism advocacy groups reacted to the one-minute teaser, slamming the decision to have Ziegler cast in the title role rather than an actor on the autism spectrum. Sia vehemently defended the film on Twitter at the time.

"@sia has got this one wrong. There are so many talented autistic actors out there," tweeted the National Autism Society, while Irish actress Bronagh Waugh wrote: "Hi Sia, can I ask why you didn't cast a disabled actor for this part? It's pretty offensive the way you've chosen to portray this character. People with disabilities are not broken and don't need fixing."

"I agree," Sia responded to Waugh. "I've never referred to music as disabled. Special abilities is what I’ve always said, and casting someone at her level of functioning was cruel, not kind, so I made the executive decision that we would do our best to lovingly represent the community."

Sia continued to defend the film on Twitter, saying that she spent three years researching the film's subject matter. "None of you have even seen it. Such a bummer!" she wrote to another Twitter user questioning the trailer.

Music is co-written by Sia and Dallas Clayton, and she also penned the soundtrack to the movie, which additionally stars Leslie Odom Jr., Juliette Lewis and Ben Schwartz.

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"I cast thirteen neuroatypical people, three trans folk, and not as f------ prostitutes or drug addicts but as doctors, nurses and singers," Sia tweeted at the time. "F----- sad nobody's even seen the dang movie. My heart has always been in the right place."

Elsewhere in her tweets, Sia said the character of Music is "based completely on my neuro atypical friend. He found it too stressful being non verbal, and I made this movie with nothing but love for him and his mother." She added that she "did try" to consult actors on the spectrum but decided that it "felt more compassionate to use Maddie. That was my call."

"Oh my goodness you haven't even seen the film!! Please give us a chance. Our intentions are good, meaningful, loving," Sia wrote in another response, adding in a separate tweet: "I believe this movie is beautiful, Will create more good than harm and if I’m wrong I’ll pay for it for the rest of my life."

Sia did not respond to PEOPLE's request for comment at the time. In another post, Sia wrote that "mocking is the very last thing either Maddie or I wanted to be so misunderstood. Please watch it before you judge it."

In December, Ziegler talked to ELLE about making Music and called the experience “incredible.”

“I have always dreamt of doing movies and now that it's become a reality,” the dancer said. “Just getting to work with some of the people that I did, I was only in awe. Sia, Kate Hudson, Leslie Odom Jr. — all of them I look up to so much. I filmed [the movie] when I was 14 and now being 18 looking back I'm like, I cannot believe I did that."