Robin and Barney have made a slight change to their wedding guest list and they're in for a treat, as they have replaced Erin Hannon with Sarah Newlin.

'True Blood' and 'Pitch Perfect' actress Anna Camp is set to make an appearance on Season 9 of 'How I Met Your Mother.' Camp will play Cassie, an attractive wedding guest who turns out to be a nuisance for Ted. The role was originally given to Ellie Kemper (Erin from 'The Office'), who backed out due to a scheduling conflict, TVLine reports.

Camp has been very busy these days. She recently reprised her role as villainess Sarah Newlin in Season 6 of 'True Blood,' and has been cast in TBS' new sitcom 'Ground Floor,' which stars two of her fellow 'Pitch Perfect' cast members, Alexis Knapp and Skylar Astin. A 'Pitch Perfect' sequel is set to hit theaters in 2015.

The 30-year-old actress has had guest arcs on 'The Good Wife' and 'Mad Men,' and she started out as a series regular on Season 1 of 'The Mindy Project.' Camp also appeared in 2011's 'The Help' and starred in the 2008 Broadway revival of 'Equus.'

Season 9 of 'How I Met Your Mother', which will be the show's last, premieres on Sept. 23. The nuptials of Robin and Barney and the introduction of the mother are some of the most highly anticipated moments from the CBS sitcom's final season. In the Season 8 finale, the mother (Ted's future wife) was revealed on screen for the first time, played by Cristin Milioti. 

'How I Met Your Mother' stars Josh Radnor, Neil Patrick Harris, Jason Segel, Alyson Hannigan and Cobie Smulders.

A new promo for Season 9 shows that the wedding may not go as smoothly as planned. See the clip below.