Help with Lua OO

[mtp1032]

Here is an error I'm having trouble tracking down.

Message: Interface\AddOns\SandBox\libs\UnitTests.lua:38: attempt to call method 'is_a' (a string value)

Here's my class declaration followed by the _init(...) definition

Slot = {}
Slot.__index = Slot
 
setmetatable(Slot, {
    __index = Container,        -- makes the inheritance work
     __call = function (cls, ...)
    local self = setmetatable({}, cls)
    self:_init(...)
    return self
  end,
})
 
-- This is the _init function.
function Slot:_init( bagNumber, slotIndex )
            
    Container._init(self)               -- call the base class constructor
    self.is_a = "Slot"                   -- in the parent class, this is set to "Virtual Container"
end
 
-- the is_a() method
function Slot:is_a()
   return self.is_a
end

Now, here's the test code that elicits the error:

local s = Slot(1,1)
DEFAULT_CHAT_FRAME:AddMessage( s:is_a() )

It fails with the message shown above. Both function and property work as expected in the base class.

Any thoughts?