attempt to index nil with 'LeftLowerLeg'? What does this mean?

I'm making ice skates for my game and I'm trying to get them on the player but this pops up, "attempt to index nil with 'leftlowerleg'. I don't get it. Here is the part of my script that has it.

                local Y = Instance.new("Weld")
                Y.Part0 = player.Character.LeftLowerLeg
                Y.Part1 = g.Middle
                Y.C0 = CFrame.new(0, 0, 0)
                Y.Parent = Y.Part0

here is the whole script

local Skate = workspace.IceSkates
game.Players.PlayerAdded:connect(function(player)
    local g = Skate.Leg1:clone()
        g.Parent = player.Character
        local C = g:GetChildren()
        for i=1, #C do
            if C[i].className == "Part" then
                local W = Instance.new("Weld")
                W.Part0 = g.Middle
                W.Part1 = C[i]
                local CJ = CFrame.new(g.Middle.Position)
                local C0 = g.Middle.CFrame:inverse()*CJ
                local C1 = C[i].CFrame:inverse()*CJ
                W.C0 = C0
                W.C1 = C1
                W.Parent = g.Middle
            end
                local Y = Instance.new("Weld")
                Y.Part0 = player.Character.LeftLowerLeg
                Y.Part1 = g.Middle
                Y.C0 = CFrame.new(0, 0, 0)
                Y.Parent = Y.Part0
        end

        local h = g:GetChildren()
        for i = 1, # h do
            h[i].Anchored = false
            h[i].CanCollide = false


        local g = Skate.Leg2:clone()
        g.Parent = player.Character
        local C = g:GetChildren()
        for i=1, #C do
            if C[i].className == "Part" then
                local W = Instance.new("Weld")
                W.Part0 = g.Middle
                W.Part1 = C[i]
                local CJ = CFrame.new(g.Middle.Position)
                local C0 = g.Middle.CFrame:inverse()*CJ
                local C1 = C[i].CFrame:inverse()*CJ
                W.C0 = C0
                W.C1 = C1
                W.Parent = g.Middle
            end
                local Y = Instance.new("Weld")
                Y.Part0 = player.Character.RightLowerLeg
                Y.Part1 = g.Middle
                Y.C0 = CFrame.new(0, 0, 0)
                Y.Parent = Y.Part0
        end

        local h = g:GetChildren()
        for i = 1, # h do
            h[i].Anchored = false
            h[i].CanCollide = false
        end
    end
end)