missing a upvalue set nil at start of sub function

[viruscamp]

local a, b
function f()
    a,b=nil
    return false
end

luadec issue15_fn_upvalue.lua

-- function num : 0
local a, b = nil, nil
f = function()
  -- function num : 0_0 , upvalues : a, b
  a = nil
  return false
end