How would I connect a constraint to BodyBackAttachment?

Hello there, I currently have this script:

local a = game.Workspace.a.Attachment

script.Parent.Touched:connect(function(player)
    local r = Instance.new("RopeConstraint", a.Parent)
    r.Attachment0 = r
    r.Attachment1 = player.Torso.BodyBackAttachment
    r.Length = 10
end)

This script doesn't work. Can anyone explain why, and how I can fix it?