Model doesn't weld to anything but humanoid root part in script. Why?

I am trying to weld a backpack to the characters upper torso so it can move along with the rest of the body. The only problem is every time I try to do that, the backpack spawns unwelded, about 30 studs away from my characters back. When I weld it to HumanoidRootPart, it welds, and it is in the middle of my character, but does not follow animations.

game.Players.PlayerAdded:connect(function(player)
	player.CharacterAdded:connect(function(char)
	
			local torso = char:WaitForChild("HumanoidRootPart")
			local uppertorso = char:WaitForChild("UpperTorso")--You can change this to another body part
			game:WaitForChild("ReplicatedStorage")
			local backpack = game.ReplicatedStorage.backpack:Clone()
			backpack.Parent = char

			local weld = Instance.new("Motor6D", uppertorso) --Could also use a Motor6D, or likewise
			weld.C0 = CFrame.new() --Both C0 and C1 are, by default, set to a blank CFrame
			weld.C1 = CFrame.new()
		weld.Enabled = true
		weld.Part0 = backpack.PrimaryPart

		
		weld.Part1 = uppertorso
			player:WaitForChild("leaderstats")
			backpack.textpart.SurfaceGui.TextLabel.Text = "DNA: "..tostring(player.leaderstats.DNA.Value)
		end)

		end)