hp_laserjet_m506 cannot get status

Hi,

- Ubuntu 16.04
- hp_laserjet_m506
- hpmud_open_device OK
- hpmud_open_channel failed,the channel is HPMUD_S_EWS_LEDM_CHANNEL, it returns HPMUD_R_INVALID_SN

sudo hp-plugin -d

message:
HP Linux Imaging and Printing System (ver. 3.16.3)
Supply Levels Utility ver. 2.0

Copyright (c) 2001-15 HP Development Company, LP
This software comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to distribute it
under certain conditions. See COPYING file for more details.

hp-levels[31373]: debug: getDeviceUri(None, None, ('hp',), None, , True)
hp-levels[31373]: debug: Mode=0
hp-levels[31373]: debug: Cache miss: hp_laserjet_m506
hp-levels[31373]: debug: Reading file: /usr/share/hplip/data/models/models.dat
hp-levels[31373]: debug: Searching for section [hp_laserjet_m506] in file /usr/share/hplip/data/models/models.dat
hp-levels[31373]: debug: Found section [hp_laserjet_m506] in file /usr/share/hplip/data/models/models.dat
hp-levels[31373]: debug: Exception: 4 (Unknown/invalid device-uri field)
hp-levels[31373]: debug: Exception: 4 (Unknown/invalid device-uri field)
hp-levels[31373]: debug: {'hp:/usb/HP_LaserJet_M506?serial=PHBXJ4D1DW': ['ccis-printer']}
Using device : hp:/usb/HP_LaserJet_M506?serial=PHBXJ4D1DW

hp-levels[31373]: debug: {'type': 1, 'kind': 4, 'level': 99, 'health': 0, 'level-trigger': 0, 'agent-sku': ''}

hp-levels[31373]: debug: 1: agent_type 1 agent_kind 4 agent_sku 'CF287A/CF287X'
Black toner cartridge
Part No.: CF287A/CF287X
Health: Good/OK

------------------------------------------------------------------------------------------------------------------------
|//////////////////////////////////////////////////////////////////////////////////////////////////////////////////////| (approx. 99%)
------------------------------------------------------------------------------------------------------------------------

Done.

- what's the "Unknown/invalid device-uri field" meaning?

-i'm using HP-EWS-LEDM channel in libhpmud-dev ,but the function hpmud_open_channel() returns HPMUD_R_INVALID_SN.i dont know why.