flake8#Flake8UnplaceMarkers() fail

[mscheper]

When I try :call flake8#Flake8UnplaceMarkers(), either manually or with the autocmd I set up for it (on <S-F7>), I get the following error:

Error detected while processing function flake8#Flake8UnplaceMarkers[1]..<SNR>79_UnplaceMarkers:
line    9:
E121: Undefined variable: s:markerdata
E116: Invalid arguments for function values(s:markerdata)
E15: Invalid expression: values(s:markerdata)

I get an error whether let g:flake8_show_in_gutter=1, let g:flake8_show_in_file=1 or both are set.

Vim 8.0 under Mint 19, with the following additional plugins:

Plug 'jlanzarotta/bufexplorer'
Plug 'tmhedberg/SimpylFold'
Plug 'vim-scripts/indentpython.vim'
Plug 'Rigellute/shades-of-purple.vim'
Plug 'Valloric/YouCompleteMe'
Plug 'vim-syntastic/syntastic'
Plug 'nvie/vim-flake8'