Traffic policy not evaluating TCL commands

I've setup a traffic policy to check if a header exists and if it doesn't then to add it. That bit of the logic works however it doesn't add what I need it to - it basically doesn't evaluate the tcl command I put in.

I've tried it using with and without quotes and basically the output I get into the XFF header is the string so either "tcl:[IP::client_addr]" or just tcl:[IP::client_addr]

        actions {
            0 {
                http-header
                replace
                name X-Forwarded-for
                value \"tcl:[IP::client_addr]\"
            }

or actions { 0 { http-header replace name X-Forwarded-for value tcl:[IP::client_addr] }

There are good reasons why I am not using the standard Insert XFF in the HTTP profile and whilst it could easily be done by an iRule I really need this to work, as it should do, in a traffic policy.

I am running 11.5.1 HF7