check_diskio plugin runs in CLI but not in web interface


This is what I get fro the CLI

$ /usr/lib64/nagios/plugins/contrib/check_diskio -d /dev/xvda -w 4000 -c 6000
CHECK_DISKIO OK - xvda 117 sectors/s | WRITE=100;4000;6000 READ=17;4000;6000 TOTAL=117;4000;6000

On the Nagios Page, using a web browser, I get

(Service check did not exit properly)

My /etc/nagios/command-plugins.cfg has

command[check_diskio]=/usr/lib64/nagios/plugins/contrib/check_diskio -d /dev/xvda -w 4000 -c 6000

My /etc/nagios/objects/commands.cfg has

define command{
command_name check_diskio
command_line $USER1$/contrib/check_diskio --device /dev/xvda --warning 4000 --critical 6000

My /etc/nagios/objects/hosts.cfg

define service{
use local-service
host_name devsvr
service_description Disk IO
check_command check_diskio
notifications_enabled 0

All perl module dependencies of check_diskio has been installed. It is obvious that I have misconfigured it but I do not know which part is misconfigured. I hope somebody would shed some light on this. Thanks!