Trying to set a negative value for `filestore_merge_threshold` in Red Hat Ceph Storage fails with "Invalid argument", why?

Issue

• While trying to set filestore_merge_threshold = -40, the command returns an "Invalid argument".

# ceph --admin-daemon /var/run/ceph/ceph-osd.312.asok config set filestore_merge_threshold -40
{   
    "error": "error setting 'filestore_merge_threshold' to '-40': (22) Invalid argument"
}

# ceph tell osd.312 injectargs '--filestore_merge_threshold -40'
Parse error setting filestore_merge_threshold to '-40' using injectargs.