commitBuffer: invalid arguments with funky characters in description

[nmattia]

/tmp/tmp.7L1hIRcwlq$ niv add spacchetti/spago
Adding package spacchetti/spago
  Reading sources file
  Writing new sources file
Done: Adding package spacchetti/spago
/tmp/tmp.7L1hIRcwlq$ niv show spago
Showing package spago
Reading sources file
  homepage: 
  url: https://github.com/spacchetti/spago/archive/8ce19ea5648f8bd65d6db88428bac56e66cf2560.tar.gz
  owner: spacchetti
  branch: master
  url_template: https://github.com/<owner>/<repo>/archive/<rev>.tar.gz
  repo: spago
  type: tarball
  sha256: 1mnx4y4298q94iq6x613ijak16a2i0chrp5qvrv2wzl2za63p8kb
  description: niv: <stdout>: commitBuffer: invalid argument (invalid character)