Chris Stapleton had an unannounced, surprise duet parter when he took the stage to perform "Maggie's Song" at the 2021 ACM Awards: Miranda Lambert. As it turns out, Lambert was a last-minute fill-in for Stapleton's frequent back-up vocalist and wife, Morgane Stapleton.

Per People, Morgane originally planned to perform the song with Stapleton at the ACMs, but had to step away.

"Due to her prior doula commitment, Morgane Stapleton is unable to join Chris Stapleton at the Bluebird as scheduled," reads the Academy of Country Music's full statement to People. "Miranda Lambert will instead take the Bluebird stage with Chris tonight for "Maggie's Song.""

Backstage in the virtual press room, Stapleton expanded on the last-minute lineup change, revealing that his wife was actually helping welcome a new member into the Stapleton extended family.

"I think my sister-in-law's in labor right now, or may be on her way to the hospital, I'm not sure. My wife is kind of on baby duty, and for once, not her own baby," said the singer, who shares five children with Morgane.

He added that in general, he's always very grateful to be able to share the stage with his wife. "It's always very special, and I miss her very much tonight," Stapleton added. "But I was so happy that Miranda agreed to fill in, kind of last minute."

Lambert was a fitting, if last-minute, substitute, given the song's subject matter. "Maggie's Song" is a tribute to the Stapleton family's beloved, late lab-terrier mix dog, whose name was Maggie. Lambert, of course, is an animal lover who even helms her own MuttNation Foundation, an organization that supports shelter pets across the country.

It's a busy night for Lambert, who also opened the ACMs with a performance of "Drunk (and I Don't Wanna Go Home)," her duet with Elle King. She also performed "In His Arms," another new collaboration with the singer's fellow Texans and fellow songwriters Jack Ingram and Jon Randall.

Lambert is nominated in multiple categories at Sunday night's (April 18) show: She's in the running for trophies like Female Artist, Single and Song of the Year.

The 2021 ACM Awards air live on CBS and stream live on Paramount+. Sign up for Paramount+ here.

See Pictures From the 2021 ACM Awards Red Carpet:

The best dressed on the 2021 ACM Awards red carpet brought color — lots of color — for an awards show that promises to be a celebration. Photos show stars like Carly Pearce, Kelsea Ballerini, Chris Young, Kane Brown and more wearing the hottest and often most expensive designer outfits, or suits they designed themselves, in the case of Jimmie Allen.

Check out a full gallery of country music's hottest stars on the ACM Awards red carpet. Who do you think is best dressed and worst dressed?

The 2021 ACM Awards took place at the Bluebird Cafe, Grand Ole Opry and Ryman Auditorium in Nashville on Sunday (April 18).