pip and conda environment.yml: unsupported operand type(s) for +: 'NoneType' and 'list'

I am trying to install a package with pip inside a conda env. So I have an environment.yml file as follows:

name: test-env

dependencies:
    - pip:
        - "git+https://github.com/choldgraf/download"

But when I run conda env update --file environment.yml, I get:

Using Anaconda API: https://api.anaconda.org
Fetching package metadata .............
Solving package specifications: An unexpected error has occurred.
Please consider posting the following information to the
conda GitHub issue tracker at:

# Here some configuration that I omit    

Traceback (most recent call last):
  File "/home/mathurin/anaconda3/lib/python3.5/site-packages/conda/exceptions.py", line 634, in conda_exception_handler
    return_value = func(*args, **kwargs)
  File "/home/mathurin/anaconda3/lib/python3.5/site-packages/conda_env/cli/main_update.py", line 106, in execute
    installer.install(prefix, specs, args, env, prune=args.prune)
  File "/home/mathurin/anaconda3/lib/python3.5/site-packages/conda_env/installers/pip.py", line 8, in install
    pip_cmd = pip_args(prefix) + ['install', ] + specs
TypeError: unsupported operand type(s) for +: 'NoneType' and 'list'

However, just typing pip install git+https://github.com/choldgraf/download in my bash console works fine. What am I doing wrong ?

EDIT: my first thought was to update conda. I am now at version 4.3.23, and trying to conda update conda yields:

# All requested packages already installed.
# packages in environment at ~/anaconda3:
#
conda                     4.3.23                   py35_0    conda-forge