[CentOS] NTP error message on /var/log/messages

I just setup CENTOS 4.7 with latest patches on DELL server.  I also configured NTP point to out time server.  I found /var/log/messages file every 20 to 30 minutes will generate a error message :

Mar 15 14:28:15 SER1 ntpd[25037]: sendto(172.29.21.16): Invalid argument
Mar 15 14:45:22 SER1 ntpd[25037]: sendto(172.29.21.16): Invalid argument
Mar 15 15:02:29 SER1 ntpd[25037]: sendto(172.29.21.16): Invalid argument

Any one know whar wring it is?

Thanks



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