Systemmonitor disk_use unavailable

Configuration
1

This is my df -h

dev             674M     0    674M   0% /dev
run             935M  7,0M    928M   1% /run
/dev/sda2        15G   13G    1,1G  93% /
tmpfs           935M  1,8M    933M   1% /dev/shm
tmpfs           935M  4,0K    935M   1% /tmp
/dev/sda1       200M   40M    161M  20% /boot
/dev/sdb2        28G   25G    2,2G  92% /mnt
tmpfs           187M  8,0K    187M   1% /run/user/1001

I wanted to check /dev/sdb2 like this

- type: disk_use_percent
        arg: /dev/sdb2

It’s unavailable

What do I have to do?

2

Maybe try the mountpoint?

arg: /mnt

EDIT: Oh, and in yaml indentation is essential.
This is how it’s got to be:

sensor:
  - platform: systemmonitor
    resources:
      - type: disk_use_percent
        arg: /srv
3

Already tried, arg: /mnt gives me /dev/sda2 as output.