Can not access flow variables related to TLS

[#1]

I have configured the two way TLS. I want to get the client certificate serial number in javascript policy and from the documentation it seems that there are certain flow variables available including tls.client.cert.serial but when I try to read this variable in javascript policy using

sn = context.getVariable("tls.client.cert.serial")

the value of sn is null. On the other hand I can read the client.cn successfully using

cn = context.getVariable("client.cn")

This page https://docs.apigee.com/api-platform/system-administration/tls-vars#capturing-tls-connection-informa... say that flow variables are available but I am not able to read them. Is there a specific way to read them?

Thanks