list index out of range when using g:easytags_by_filetype

[bonds]

I'm trying to generate tags for Haskell files, but I'm getting this error:

easytags.vim 3.11: Asynchronous tags file update failed! (Vim(let):E684: list index out of range: 4 at function xolox#misc#async#inside_child[15]..xolox#easytags#update#with_vim[10]..<SNR>4_save_by_filetype, line 5)

My config looks like this:

let g:easytags_cmd = '/usr/local/bin/ectags'
let g:easytags_by_filetype = '~/.vim/tags'
let g:easytags_async = 1

let g:easytags_languages = {
\   'haskell': {
\     'cmd': 'hasktags',
\     'args': [],
\     'fileoutput_opt': '-f',
\     'stdout_opt': '-f-',
\     'recurse_flag': '-R'
\   }
\}

Turning async off doesn't help, I still get that error when I :UpdateTags. Turning off g:easytags_by_filetype does help, and easytags works fine if I do that.