the border of fzf-preview window is broken

[zl03jsj]

the border of fzf-preview window is broken, how can i fix this:

all config is default:

local M = {
    'junegunn/fzf.vim',
    branch = "master",
    dependencies = {
        'junegunn/fzf',
        branch = "master",
        build = "./install --all"
    }
}
M.config = function()
    vim.api.nvim_set_keymap('n', '<leader>sa', ':RG<cr>',       { silent = true, noremap = true })
    vim.api.nvim_set_keymap('n', '<leader>sf', ':Files<cr>',    { silent = true, noremap = true })
    vim.api.nvim_set_keymap('n', '<leader>sb', ':Buffers<cr>',  { silent = true, noremap = true })
end
return M