Openvpn service failed to start on CentOS 5.5

[sohel]

When I run

#service openvpn start , I get the error message : starting openvpn [failed]. I checked the log from /var/log/messages and found : "openvpn[4676]: Options error: Parameter ca_file can only be specified in TLS-mode, i.e. where --tls-server or --tls-client is also specified".

Help appreciated to solve it.

Thanks.