Alsamixer won't work with bluetooth device

Alsamixer won't work with bluetooth device

I'm running Debian unstable. My .asoundrc looks like this:
pcm.btheadset
{
type plug
slave
{
pcm
{
type bluetooth
device 5A:5A:5A:A6:08:09
profile "auto"
}
}
}
ctl.btheadset
{
type bluetooth
}
I can play music through the headset, however I cannot control the volume.
$ alsamixer -D btheadset
ALSA lib audio/ctl_bluetooth.c:167:(bluetooth_send_ctl) Unable to receive
new volume value from server
ALSA lib audio/ctl_bluetooth.c:161:(bluetooth_send_ctl) Unable to request
new volume value to server: Broken pipe
cannot load mixer controls: Broken pipe
daemon.log has this:
bluetoothd[15628]: Invalid message: length mismatch
Any ideas? I suspected this may be some mismatch of binaries, so I tried
downgrading bluez to Debian stable. No luck. Maybe I should try the same
with alsa libs...
A lot of FAQs and tutorials suggest that PulseAudio should automatically
solve this, however I installed it, it pulled down dozens of dependencies
I have no interest in, and turned out to be a very user-hostile daemon
that refused to play any sound at all. So I am not interested in that as a
solution.