[alsa-devel] alsa conf file

[tomars]

I write a asound.conf file in the /etc/ , the detail is as follows.
pcm.hardware{
        type hw:0
        card 0
        device 0
}
pcm_slave.test {
        pcm "hardware"
        format S16_LE
        rate 44100
        channels 2
        period_time 20000
        buffer_time 80000
}
pcm.mix1{
        type dmix
        ipc_key 1235675
        ipc_key_add_uid true
        slave "test"
}

pcm.mix2{
        type dmix
        ipc_key 1235675
        ipc_key_add_uid true
        slave {
                pcm "hw:0"
                format S16_LE
                rate 44100
                channels 2
                period_time 20000
                buffer_time 80000
        }
}
pcm.!default {
        type plug
        slave.pcm mix2
		#slave.pcm mix1
}

when I use the slave.pcm mix2, the aplay works well. but if I select the slave.pcm mix1, the aplay gets the info:
ALSA lib pcm_direct.c:1462:(_snd_pcm_direct_get_slave_ipc_offset) Invalid type 'dmix' for slave PCM

I compare the definition of mix1 with the definition of mix2, I feel they have the identical meaning. so why does the mix2 get the error?
thank you!