Unknown module "clients"

[Stefano Zanmarchi]

Hello,
I'm performing some filtering, based on the client, in the authorize
section of inner-tunnel:

authorize {
        preprocess
        rewrite.calling_station_id
        if ("%{clients:shortname}" =~ /internal_radius_(.*)/i) {
          ok
          update control {
            Auth-Type := Accept
          }
        }
...
}

The output logs in debug mode show the following warning:
..
++? if ("%{clients:shortname}" =~ /internal_radius_(.*)/i)
WARNING: Unknown module "clients" in string expansion "%"
..

Does this mean that the 'if ("%{clients:shortname}"...' clause is not being
evaluated?
How can I solve the problem generating the warning?

Thank you,
Stefano