Though they may seem invincible onstage at times, not all musicians are at 100 percent all the time. Such was the case for Amon Amarth vocalist Johan Hegg, who had to sit out the band's Saturday night performance after temporarily loosing his voice. Luckily, Huntress frontwoman Jill Janus was around to lend a hand.

The group played primarily an instrumental set for the fans in Plymouth, but Janus made a guest turn on the song 'Guardians of Asgaard.' Fan shot video of the performance can be seen above.

Hegg offered the following comment via Twitter:

Janus then followed with a tweet of her own:

Despite not being 100 percent, Hegg says he expects to return to the stage for their show Sunday night in Bristol. The two bands are currently on tour in the U.K. with Savage Messiah rounding out the touring bill.