Invalid argument error when setting option with vim.opt.clipboard:append

[macintacos]

• nvim --version: NVIM v0.5.0-dev+1381-gb3e3ab056
• Operating system/version: macOS 11.4
• Terminal name/version:
• $TERM: iTerm 3.4.7beta2

Steps to reproduce using nvim -u NORC

nvim -u NORC
# Alternative for shell-related problems:
# env -i TERM=ansi-256color "$(which nvim)"

# Run the following command
:lua vim.opt.clipboard:append {"unnamedplus"}

Actual behaviour

After running the above command (which should work, according to :h lua-vim-options?) I get the following error:

E5108: Error executing lua ...ovim/HEAD-b3e3ab0_2/share/nvim/runtime/lua/vim/_meta.lua:170: E474: Invalid argument

Expected behaviour

The option should be set without issue.

[macintacos]

Just confirmed that prepend works as expected, so not sure what's going on here:

vim.opt.clipboard:prepend {"unnamedplus"}

[ChrisTsou]

Probably related:
vim.opt.clipboard:append("vert:▏")
causes:

E5113: Error while calling lua chunk: vim/shared.lua:423: s: expected string, got nil
stack traceback:
        vim/shared.lua:423: in function 'trim'
        /usr/share/nvim/runtime/lua/vim/_meta.lua:497: in function 'convert_value_to_lua'
        /usr/share/nvim/runtime/lua/vim/_meta.lua:581: in function 'add_value'
        /usr/share/nvim/runtime/lua/vim/_meta.lua:697: in function '__add'
        /usr/share/nvim/runtime/lua/vim/_meta.lua:693: in function 'append'
        /home/chris/.config/nvim/init.lua:19: in main chunk

vim.o.clipboard = vim.o.clipboard .. vert:▏ works