Webclient Plugin loading fails with error : "java.lang.NoSuchFieldError: sessionKey"

I've successfully registered a plugin (developed using 6.0 sdk) on a vCenter 5.5 U3. I'm able to access the plugin shortcut as well from the vCenter GUI. But when I try to access the "sessionKey" field of "ServerInfo" class, it fails with error "Handler processing failed; nested exception is java.lang.NoSuchFieldError: sessionKey] with root cause java.lang.NoSuchFieldError: sessionKey".