July 17, 2018 04:11 PM

Whoops! Cher admitted she had to keep a close eye on her outfit to avoid a fashion mishap at Monday night’s premiere of Mamma Mia 2: Here We Go Again in London.

The star, who appears in the new Mamma Mia movie as the grandmother to Amanda Seyfried’s character, wore a lacy bodysuit beneath a long black trench coat and wide leg pants, but according to Cher, the sheer undergarment wasn’t the one she was meant to be wearing that night.

“I am actually wearing the wrong bodysuit. They sent the wrong bodysuit!” Cher laughed on the red carpet during an interview with Access Hollywood.

She added, “It is one size too small and I keep saying to everybody, like, ‘Amanda [Seyfried], Amanda [Seyfried], make sure my jacket is shut!’

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After the carpet, in an exclusive PEOPLE Facebook live talk with the cast, moderated by PEOPLE deputy editor Dan Wakeford, Cher, 72, opened up about the small age gap between herself and Meryl Streep, 69, who portrays her daughter in the film.

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“That doesn’t make any difference,” she said. “I was thrilled to death, and it was so silly. It was great fun, and I love her [Meryl], so it was perfect.”

Cher continued, “It was great, and she was hiding watching me sing, and I was so excited. I didn’t find out until afterwards—I would have been more nervous.”

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In the follow-up to the 2008 adaptation, a pregnant Sophie (Seyfried) calls on the help of her mother’s friends and former bandmates Tanya and Rosie as she prepares for motherhood. As the film switches between the past and present, Lily James stars as Streep’s character in her younger years.

Mamma Mia! Here We Go Again hits theaters Friday, July 20.

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