How to keep a local function private in a zsh `autoload` module?

I want to define a helper function in a zsh autoload-ed module. The problem is, if I just define in my my_autoloaded_fn file such as

function helper {
  # ...
}

helper a
helper b

the name helper will become defined in the global namespace once my_autoloaded_fn is invoked. I tried also

local helper
function helper {
  # ...
}

but the local declaration seems to have no effect. And neither local -f works.