Changing metatable in Lua breaks colon operator

Changing metatable in Lua breaks colon operator

While learning Lua, I borrowed some code from here to use string indexing,
which is exactly this:
getmetatable("").__index = function(str, i) return string.sub(str, i, i) end
After that, I wrote a function to reverse a string as practice.
function reverse_string(str)
local s = ""
for i = string.len(str), 1, -1 do s = s .. str[i] end
return s
end
That works fine, until I change the string.len(str) to str:len(), then I
get this error:
reverse.lua:9: bad argument #2 to 'sub' (number expected, got string)
Debugging print()'s tell me that the __index function is being called on
str:len(), and that the i argument is becoming the string "len". I know
that str:len() works without the metatable there, but as soon as I add it
this happens, why?