The Forgetting Sarah Marshall star told DeGeneres all about a recent meltdown when fiance Dax Shepard hired a sloth for her birthday party.
She revealed she was so overcome with emotion at the thought of coming face to face with her favourite animal, she suffered a panic attack.
Bell even showed DeGeneres home video footage shot by Shepard of the moment she realised she'd be sharing her party with a sloth - she started sobbing uncontrollably out of sheer excitement.
And the actress almost repeated the emotional meltdown on TV after DeGeneres hinted she had "the cutest sloth" backstage.
Desperately trying to contain herself, stunned Bell said, "My heart is beating so fast," after DeGeneres pranked her.
She added, "Why would you do that...? Oh wow!"