Anyone know why this isn't welding properly? Check description what this code is suppose to do

The torso is supposed to be welded to the torso the other way, so it is the same length as the torso.

function weld(a, b)
    local weld = Instance.new("Weld", a)
    weld.Part0 = a
    weld.Part1 = b
    weld.C0 = (a.CFrame:inverse() * b.CFrame)
    return weld
end

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