Error: Expected ‘:’ not ‘.’ calling member function (?)

I was just using metatables and instances to add custom properties and methods. I put the __index of the metatable to the instance that I'm trying to add extra properties to. But when I try to call a method such as customClass:FindFirstChild("SomeObjectName") I get this error:

Expected ':' not '.' calling member function FindFirstChild

(And yes, I called the function with a :)

My understanding is that there is something wrong with Indexing in lua so when it doesnt find the function insied the metatable it looks through the __index value and finds a function, and it calls it with the . instead of :.

I also tried doing customClass.FindFirstChild(customClass, "SomeObject") and the same error occured.

How would I fix this without adding all the indexes in the metatable then calling the functions in there?

Example of what I want to avoid having to do:

local _instance = Instance.new("Part")
local customClass = {}
customClass.__index = _instance

function customClass.FindFirstChild(name)
  return _instance:FindFirstChild(name)
end

Any help is appreciated! If you need more information, just ask!

Not sure what you're doing wrong but I've tried your code, and it seems to be working correctly

Code:

local _instance = Instance.new("Part")

--// insert a tab into our part for our new function to find! :)
local _tab = Instance.new("BoolValue",_instance)
_tab.Name = "Blah"
local customClass = {}
customClass.__index = _instance

function customClass.FindFirstChild(name)
	return _instance:FindFirstChild(name)
end

local Chk = customClass.FindFirstChild("Blah")
print(Chk,typeof(Chk),type(Chk), Chk.Parent) -- prints(Blah Instance userdata Part)