Pre-loader script not working

This preloader script I made isn’t working, and it’s outputting, " [Unable to cast to Array]". How can I fix that?

CODE
local ContentProvider = game:GetService("ContentProvider")

local assets = Instance.new("Folder", script) -- create a folder called assets in the script.
assets.Name = "Assets"

local animations = Instance.new("Folder", assets) -- create a folder called animations inside the assets folder
animations.Name = "Animations"

local animationIDs = {
	"rbxassetid://5645167603" -- animation ids to be loaded
}

for i = 1, #animationIDs do -- loop thorugh all the ids and create animation instances for them
	local animation = Instance.new("Animation")
	animation.Name = "Animation"..i
	animation.AnimationId = animationIDs[i]
	animation.Parent = animations -- add this animation to the animations folder inside assets
end

warn("Preloading assets...")
for _, content in pairs(assets:GetChildren()) do -- loop through all of the content (animations)
	for __, asset in pairs(content:GetChildren()) do -- loop through all of the content (in this case animations) and preload them
		ContentProvider:PreloadAsync(asset) -- error is here, but this is a child of assets, I don't know why it's error'ing.
	end
end
print("Preloaded assets!")

Instead of preloading every asset individually, you should just load the entire asset folder.
PreloadAsync yields until it’s done loading the assets, so you can just replace

for _, content in pairs(assets:GetChildren()) do -- your code
	for __, asset in pairs(content:GetChildren()) do
		ContentProvider:PreloadAsync(asset)
	end
end

with

ContentProvider:PreloadAsync({assets})

Side note. The reason why your code errored was because PreloadAsync takes an array of objects, not just a single instance.