Why UnknownMethod "PrepareForShutdown"

$ dbus-send --print-reply --system /org/freedesktop/login1 org.freedesktop.login1.Manager.PrepareForShutdown boolean:true
Error org.freedesktop.DBus.Error.UnknownMethod: Method "PrepareForShutdown" with signature "b" on interface "org.freedesktop.login1.Manager" doesn't exist

$ dbus-monitor --system
signal time=1614851344.213784 sender=org.freedesktop.DBus -> destination=:1.27062 serial=2 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameAcquired
   string ":1.27062"
signal time=1614851344.213880 sender=org.freedesktop.DBus -> destination=:1.27062 serial=4 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameLost
   string ":1.27062"
method call time=1614851347.546670 sender=:1.27063 -> destination=org.freedesktop.DBus serial=1 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=Hello
method return time=1614851347.546689 sender=org.freedesktop.DBus -> destination=:1.27063 serial=1 reply_serial=1
   string ":1.27063"
signal time=1614851347.546702 sender=org.freedesktop.DBus -> destination=(null destination) serial=57054 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameOwnerChanged
   string ":1.27063"
   string ""
   string ":1.27063"
signal time=1614851347.546719 sender=org.freedesktop.DBus -> destination=:1.27063 serial=2 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameAcquired
   string ":1.27063"
signal time=1614851347.547641 sender=org.freedesktop.DBus -> destination=:1.27063 serial=5 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameLost
   string ":1.27063"
signal time=1614851347.547665 sender=org.freedesktop.DBus -> destination=(null destination) serial=57055 path=/org/freedesktop/DBus; interface=org.freedesktop.DBus; member=NameOwnerChanged
   string ":1.27063"
   string ":1.27063"
   string ""