As Michael Bublé explained at Madison Square Garden this week, he never knows what's going to happen when he asks a fan to sing during one of his concerts. What happened at Wednesday night's show in New York City stunned him and everyone else.

While talking about songs that people like to sing in the shower, he began interacting with the audience.

Allison Bellucci was in the front row and explained that she just knew he was going to ask someone about their shower song, so she started yelling that her sister, Erin, can sing.

When Bublé walked over to the sisters, Erin told him that her shower song was "At Last."

He then handed the 21-year-old University of Deleware student the microphone and she got to sing her rendition of the song.

Erin's sister recorded it all on her phone as she stunned everyone in the crowd. Even Bublé seemed genuinely surprised, getting wide-eyed during parts of her impromptu performance.

Michael Buble shocked at fans singing
Michael Bublé reacts to a fan singing at his New York City concert on Wednesday, Feb. 20, 2019.
Allison Bellucci

He later remarked how it may not be her last time singing at Madison Square Garden.

"That was incredible! Too incredible if you ask me," Bublé joked afterwards.

Michael Buble fan singing Erin Bellucci
Erin Bellucci, 21, sings "At Last" during Michael Bublé's concert at Madison Square Garden on Feb. 20, 2019.
Allison Bellucci

It was so good that the legendary singer stopped to explain to the audience that they don't set up the fan performances ahead of time, which can sometimes create real horror.

"I never know what's going to happen. Some nights that doesn't happen. Did you guys see that," he marveled.

Just this week a former Texas A&M student went viral after singing "Fly Me to the Moon" at Buble's tour stop in Georgia.