Can't get appsession setting in HAProxy to work

My haproxy.cfg has a frontend which then redirects to a bunch of backends,

here is a sample backend (note that connections into this have already been through stunnel to rip out SSL:

backend thenameofit        
reqadd          X-Forwarded-Proto:\ https
reqadd          FRONT_END_HTTPS:\ on
server myserver1 serverhostnamex:80 weight 1 maxconn 5000 check
server myserver2 serverhostnamey:80 weight 1 maxconn 5000 check
appsession ASP.NET_SessionId len 100 timeout 3h

I'm sure I'm simply missing a setting somewhere, but I must be missing it when reading the config documentation.

I'm using HAProxy 1.4.13.