Zsh: completion not working for self-defined function based on git

In my .zshrc file I have written a function based on git log:

zsh
lg() {
    git log \
    --abbrev=12 \
    --graph \
    --oneline \
    --color=always \
    --format="%C(cyan)%h %C(blue)%ar %C(auto)%d %C(yellow)%s %C(white)%ae" "$@" |
    fzf

To enable autocompletion for this function I have added to my .zshrc:

zsh
compdef _git-log lg

The _git-log function is indeed the basic completion function for git log in zsh. It is inside /usr/share/zsh/functions/Completion/Unix/_git. But using this method as recommended [here](https://unix.stackexchange.com/questions/269791/zsh-tab-completion-for-function-with-git-commands) , doesn't work on my side. After sourcing my .zshrc and try to use my lg command I keep getting in my terminal:

>$ lg (eval):1: command not found: _git-log

How to use completion functions from completions file for other function? Edit: As suggested by @Stéphane Chazelas , I used the load_helper_functions from @Gille's answer . At the end of my .zshrc, I have now :

zsh
load_helper_functions () {
    emulate -LR zsh
    if ((!$+functions[$1])) || [[ $functions[$1] = 'builtin autoload'* ]]; then
        autoload +X +U $1
        functions[$1]=${functions[$1]%$'\n'*}
        $1
  fi
}
load_helper_functions _git
compdef _git lg=git-log