Custom branch rule expression not working as expected

I am trying to create a custom branch in an APM policy. I have a custom session variable that has been set in a prior "Variable Assign" step, but the policy never follows my custom branch....it always goes to fallback.

The branch rule is expression is: expr {[string match "webmail*" [mget {session.custom.hostname}]] }

Basically, what I want is the user to get directed to our webmail service if the hostname they used to connect started with "webmail". If not, they should get the webtop with a selection of services.

Can you seen anything wrong with my expression above? I have also tried simpler versions that I think should work, like expr {string match "webmail*" [mget session.custom.hostname] }