Why can't I play this animation in this NPC? (Have checked other questions)

I am trying to run an animation in an NPC but I keep getting the same error, this is my code:

Wizard = {game.Workspace.ClassChoose.WizardSet.WizardDummy, 7, 15}
function Choose(Class)
	if Class == "Mage" then
		local animController = Instance.new("AnimationController")
		animController.Parent = Wizard[1]
		local animTrack = animController:LoadAnimation("http://www.roblox.com/Asset?ID=371493384")
		animTrack:Play()		
		Wizard[1].Tome.Book.Transparency = 0
		Wizard[1].Tome.Pages.Transparency = 0		
		for i = 1, 10 do
			local Magic = game.Lighting.Magic:Clone()
			Magic.Position = CFrame.new(Wizard[0]["Left Arm"].Position + (math.random(5, 10)/100))
		end
	end
end

Choose("Mage")

Why does this return this error:

23:02:36.087 - Unable to cast value to Object

23:02:36.087 - Script 'Players.Player.PlayerGui.LocalScript', Line 7 - global Choose

23:02:36.088 - Script 'Players.Player.PlayerGui.LocalScript', Line 19

23:02:36.088 - Stack End

Finally; I have used the array correctly, for some reason the array now counts from 1 in the first place not 0. Thanks.

EDIT: With what you have said lines 4-6 have been changed to:

	local anim = Instance.new("Animation")	
	anim.AnimationId = "http://www.roblox.com/Asset?ID=371493384"
	local animController = Instance.new("AnimationController")
	local animTrack = animController:LoadAnimation(anim)	
	anim.Parent = Classes[Class][1]
	animTrack:Play()

But I am getting the same error.