Can't load default or catalog R15 animations on an NPC?

I seem to be unable to load catalog animations or default R15 animations on an NPC character. It works just fine for R6, which confuses me. Here is my code:

local R6_DEFAULT_RUN_ANIMATION = "rbxassetid://180426354"
local R15_DEFAULT_RUN_ANIMATION = "rbxassetid://2510198475"

local description: HumanoidDescription = dummy.Humanoid:GetAppliedDescription()
local walkAnimation = Instance.new("Animation")

if description.RunAnimation ~= 0 then
    walkAnimation.AnimationId = "rbxassetid://" .. description.RunAnimation
else
    if dummy.Humanoid.RigType == Enum.RigType.R15 then
        walkAnimation.AnimationId = R15_DEFAULT_RUN_ANIMATION
    else
        walkAnimation.AnimationId = R6_DEFAULT_RUN_ANIMATION
    end
end

local animator: Animator? = dummy.Humanoid:WaitForChild("Animator")
local loaded = animator:LoadAnimation(walkAnimation)
loaded:Play()

Again, this works for R6 characters, but not R15. The NPC dummy has the same humanoid description as the LocalPlayer, if that helps.