Couldn't cast value to object animation?

Hi guys!

I have been running this piece of code fine for AGES!

Today, I started up studio and it ran fine for the first NPC, but it errored for the second - "Couldn't cast value to object".

Any ideas whats going on?

1. It's previously worked perfectly. 2. It now works with only the first NPC?!

local walk = game.Workspace.Animations.walk.WalkAnim
local animTrack = folder[ai].Humanoid:LoadAnimation(walk)
animTrack:Play()

Thanks!