Why does my weld disappear when I set Part1?

I am trying to attach any part to my character using a ManualWeld and for some reason it doesn't work.

Here's my code:

player.CharacterAdded:connect(function(character)
    local UpperTorso = character:WaitForChild("UpperTorso", 5)
    if UpperTorso then
        local weld = Instance.new("ManualWeld", UpperTorso)
        weld.Part0 = UpperTorso
        weld.Part1 = game.Workspace.Brick
    end
end)

For some reason if I remove Part1 then it appears in UpperTorso, but if I set Part1 to anything, the weld disappears from UpperTorso and doesn't work. Thanks in advance.