Why dosen't my part form a weldConstraint with the Player's Left Arm?

I have this code in a server script to grab a BasePart from ReplicatedStorage, set its location, and then connect it to the player's left arm using a WeldConstraint.

        weld = Instance.new("WeldConstraint")
        shield = game:GetService("ReplicatedStorage").wshield
        shield.Parent = player.Character["Left Arm"]
        shield.CFrame = player.Character["Left Arm"].CFrame
        shield.CFrame = (shield.CFrame * CFrame.Angles(0,math.rad(90),0))
        shield.CFrame = shield.CFrame + Vector3.new(-0.5,0,0)
        weld.Part1 = player.Character["Left Arm"]
        weld.Part0 = shield

When run, this causes the shield object to appear in the correct position, but it dosen't move with the left arm as it should. Is there an issue with this code? It's possible the problem lies somewhere else, but I'm not sure.