neovim throws E474 while processing function <lambda>52[1]..<SNR>90_update

how to reproduce: :Fern . Immediately throws error.

minimal vimrc:

if exists('+compatible') && &compatible
  set nocompatible
endif

" Disable Vim's native pack feature
set packpath=

" Clone https://github.com/lambdalisue/fern.vim in somewhere
" and specify that directory to the below
set runtimepath^=~/ghq/github.com/lambdalisue/fern.vim

filetype plugin indent on
syntax on
"----------------------------------------------------------------

call plug#begin('~/.vim/plugged')
Plug 'lambdalisue/fern.vim'
call plug#end()

"----------------------------------------------------------------
echomsg "Custom minimal vimrc has loaded"

Screenshot of error Capture

Version

:version
NVIM v0.3.4
Build type: Release
LuaJIT 2.1.0-beta3
Compilation: /usr/bin/cc -g -O2 -fdebug-prefix-map=/build/neovim-PpSRXj/neovim-0.3.4=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=1 -DDISABLE_LOG -Wdate-tim
e -D_FORTIFY_SOURCE=1 -Wconversion -O2 -DNDEBUG -DMIN_LOG_LEVEL=3 -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fdi
agnostics-color=auto -Wno-array-bounds -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -I/build/neovim-PpSRXj/neovim-0.3.4/build/config -I/build/neov
im-PpSRXj/neovim-0.3.4/src -I/usr/include -I/build/neovim-PpSRXj/neovim-0.3.4/build/src/nvim/auto -I/build/neovim-PpSRXj/neovim-0.3.4/build/include
Compiled by team+vim@tracker.debian.org

Features: +acl +iconv -jemalloc +tui
See ":help feature-compile"

   system vimrc file: "$VIM/sysinit.vim"
  fall-back for $VIM: "/usr/share/nvim"

Run :checkhealth for more info

lambdalisue / vim-fern

neovim throws E474 while processing function <lambda>52[1]..<SNR>90_update #369