Error when adding new string to errorformat in init.lua?

In Neovim I want to jump through Python traceback in the quickfix list.

To do that, I set the following option

:set errorformat+=%.%#File\ \"%f\"\\,\ line\ %l%.%#

and this works well.

What I would like now is to add this command in my init.lua, But when I add the previous line, neovim raises errors when I populate the quickfix List. This is my COMPLETE init.lua file:

vim.opt.errorformat:append { "%.%#File \"%f\"\\, line %l%.%#" }

With this config file, when I populate a quickfix list with a command:

:cex system('ls -al')

Neovim raises the following error:

E377: Invalid %- in format string

It seems neovim doesn't like the '%' character because when I set the following line vim.opt.errorformat:append('foo%f') in my init.lua, neovim raises the same error.

Do you know how to append a new string to errorformat?

edit: rephrased my message