can't load the emq_auth_pgsql when the db user is NOT "root"

OS: CentOS 6.8 x86_64
EMQTTD: 2.2

my emq_auth_pgsql.conf list below:
`## Postgre Server: 5432, 127.0.0.1:5432, localhost:5432`
`auth.pgsql.server = 127.0.0.1:5432`
`auth.pgsql.pool = 8`
`auth.pgsql.username = mqtt`
`auth.pgsql.password = mqtt`
`auth.pgsql.database = mqtt`
`auth.pgsql.encoding = utf8`
`auth.pgsql.ssl = false`

and i try to load the plugin:
$ emqttd_ctl plugins load emq_auth_pgsql

load plugin error: {emq_auth_pgsql,
{bad_return,
{{emq_auth_pgsql_app,start,[normal,[]]},
{'EXIT',
{{badmatch,
{error,
{shutdown,
{failed_to_start_child,emq_auth_pgsql,
{shutdown,
{failed_to_start_child,worker_sup,
{shutdown,
{failed_to_start_child,
{worker,1},
{error,fatal,<<"28000">>,
invalid_authorization_specification,
<<"role \"root\" does not exist">>,
[{file,<<"miscinit.c">>},
{line,<<"494">>},
{routine,
<<"InitializeSessionUserId">>}]}}}}}}}}},
[{emq_auth_pgsql_app,start,2,
[{file,"src/emq_auth_pgsql_app.erl"},{line,33}]},
{application_master,start_it_old,4,
[{file,"application_master.erl"},{line,273}]}]}}}}}

when i check the crash.log, i found this message:
>2017-07-19 15:20:59 =CRASH REPORT====
crasher:
initial call: ecpool_worker:init/1
pid: <0.1794.0>
registered_name: []
exception exit: {{error,fatal,<<"28000">>,invalid_authorization_specification,<<"role \"root\" does not exist">>,[{file,<<"miscinit.c">>},{line,<<"494">>},{routine,<<"InitializeSessionUserId">>}]},[{gen_server,init_it,6,[{file,"gen_server.erl"},{line,344}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]}
ancestors: [<0.1793.0>,<0.1791.0>,emq_auth_pgsql_sup,<0.1789.0>]
messages: [{'EXIT',<0.1795.0>,normal}]
links: [<0.1793.0>]
dictionary: []
trap_exit: true
status: running
heap_size: 376
stack_size: 27
reductions: 181
neighbours:
2017-07-19 15:20:59 =SUPERVISOR REPORT====
Supervisor: {<0.1793.0>,ecpool_worker_sup}
Context: start_error
Reason: {error,fatal,<<"28000">>,invalid_authorization_specification,<<"role \"root\" does not exist">>,[{file,<<"miscinit.c">>},{line,<<"494">>},{routine,<<"InitializeSessionUserId">>}]}
Offender: [{pid,undefined},{id,{worker,1}},{mfargs,{ecpool_worker,start_link,[emq_auth_pgsql,1,emq_auth_pgsql_cli,[{pool_size,8},{auto_reconnect,1},{host,"127.0.0.1"},{port,5432},{username,"root"},{password,[]},{database,"mqtt"},{encoding,utf8},{ssl,false},{ssl_opts,[]}]]}},{restart_type,transient},{shutdown,5000},{child_type,worker}]
>
>2017-07-19 15:20:59 =SUPERVISOR REPORT====
Supervisor: {<0.1791.0>,ecpool_pool_sup}
Context: start_error
Reason: {shutdown,{failed_to_start_child,{worker,1},{error,fatal,<<"28000">>,invalid_authorization_specification,<<"role \"root\" does not exist">>,[{file,<<"miscinit.c">>},{line,<<"494">>},{routine,<<"InitializeSessionUserId">>}]}}}
Offender: [{pid,undefined},{id,worker_sup},{mfargs,{ecpool_worker_sup,start_link,[emq_auth_pgsql,emq_auth_pgsql_cli,[{pool_size,8},{auto_reconnect,1},{host,"127.0.0.1"},{port,5432},{username,"root"},{password,[]},{database,"mqtt"},{encoding,utf8},{ssl,false},{ssl_opts,[]}]]}},{restart_type,transient},{shutdown,infinity},{child_type,supervisor}]
>
>2017-07-19 15:20:59 =SUPERVISOR REPORT====
Supervisor: {local,emq_auth_pgsql_sup}
Context: start_error
Reason: {shutdown,{failed_to_start_child,worker_sup,{shutdown,{failed_to_start_child,{worker,1},{error,fatal,<<"28000">>,invalid_authorization_specification,<<"role \"root\" does not exist">>,[{file,<<"miscinit.c">>},{line,<<"494">>},{routine,<<"InitializeSessionUserId">>}]}}}}}
Offender: [{pid,undefined},{id,emq_auth_pgsql},{mfargs,{ecpool,start_pool,[emq_auth_pgsql,emq_auth_pgsql_cli,[{pool_size,8},{auto_reconnect,1},{host,"127.0.0.1"},{port,5432},{username,"root"},{password,[]},{database,"mqtt"},{encoding,utf8},{ssl,false},{ssl_opts,[]}]]}},{restart_type,permanent},{shutdown,5000},{child_type,supervisor}]
>
>2017-07-19 15:20:59 =CRASH REPORT====
crasher:
initial call: application_master:init/4
pid: <0.1788.0>
registered_name: []
exception exit: {{bad_return,{{emq_auth_pgsql_app,start,[normal,[]]},{'EXIT',{{badmatch,{error,{shutdown,{failed_to_start_child,emq_auth_pgsql,{shutdown,{failed_to_start_child,worker_sup,{shutdown,{failed_to_start_child,{worker,1},{error,fatal,<<"28000">>,invalid_authorization_specification,<<"role \"root\" does not exist">>,[{file,<<"miscinit.c">>},{line,<<"494">>},{routine,<<"InitializeSessionUserId">>}]}}}}}}}}},[{emq_auth_pgsql_app,start,2,[{file,"src/emq_auth_pgsql_app.erl"},{line,33}]},{application_master,start_it_old,4,[{file,"application_master.erl"},{line,273}]}]}}}},[{application_master,init,4,[{file,"application_master.erl"},{line,134}]},{proc_lib,init_p_do_apply,3,[{file,"proc_lib.erl"},{line,247}]}]}
ancestors: [<0.1787.0>]
messages: [{'EXIT',<0.1789.0>,normal}]
links: [<0.1787.0>,<0.935.0>]
dictionary: []
trap_exit: true
status: running
heap_size: 610
stack_size: 27
reductions: 126
neighbours:

i create the user named 'root' in the pg db, and reload the plug, it WORKS!!!
`createuser -sP root`
`emqttd_ctl plugins load emq_auth_pgsql`
> Start apps: [emq_auth_pgsql]
Plugin emq_auth_pgsql loaded successfully.

it does NOT use the conf entry 'auth.pgsql.username'?