py3status: group module cannot be loaded

I am using the master branch. I simply put the example in the doc:

# Create a disks group that will show space on '/' and '/home'
# Change between disk modules every 30 seconds
...
order += "group disks"
...

group disks {
    cycle = 30
    format = "Disks: {output} {button}"
    click_mode = "button"

    disk "/" {
        format = "/ %avail"
    }

    disk "/home" {
        format = "/home %avail"
    }
}

to my configuration file (I also tried an empty configuration file). An error is reported:

py3status: Setup error (RuntimeError) parse_config.py line 544. Please try to fix this and reload i3wm (Mod+Shift+R)

Without group module or when the group module only contains a single module it works fine.