lua – 运算符重载不起作用

Set = {}

mt = {}
mt.__add = Set.union

--create a new set with the values of the given list
function Set.new (l) 
    local set = {}
    setmetatable (set,mt)
    for _,v in ipairs (l) do 
        set [v] = true
    end
    return set
end 

function Set.union (a,b)
    local result = Set.new {}
    for k in pairs (a) do result [k] = true end
    for k in pairs (b) do result [k] = true end
    return result
end

function Set.intersection (a,b)
    local result = Set.new {}
    for k in pairs (a) do result [k] = b[k] end
    return result
end

function Set.tostring (set)
    local l = {}
    for e in pairs (set) do
        l[#l + 1] = e
    end
    return "{" .. table.concat (l,",") .. "}"
end 

function Set.print (s)
    print (Set.tostring (s))
end 

s1 = Set.new {10,20,30,50}
s2 = Set.new {30,1}

Set.print (s1)
Set.print (s2)

s3 = s1 + s2
Set.print (s3)

但是对于Windows的最新lua我收到以下错误

lua: C:meta.lua:47: attempt to perform arithmetic on global 's1' (a table value)
stack traceback:
    C:meta.lua:47: in main chunk
    [C]: ?
{30,10,50}
{1,30}