Attempt To index nil with clone! Please help? I need to fix this error. [closed]

This is my code right here. I have no idea if it's an easy fix or not.

function commands.invisible(sender, args)
    local playerinvis = args[1]
    if playerinvis == "me" then
        local character = sender.Character
        local characterClone = character:Clone()
        characterClone.Name = tostring(sender.Name).."'s Character"
        characterClone.Parent = ServerS
        local children = character:GetChildren()
        for _, child in pairs(children) do
            if child:IsA("MeshPart") or child:IsA("Part") then
                child.Transparency = 1
                elseif child:IsA("Accessory") then
                    child:Destroy()
            end
        end
    end
end