how to mark packets

Hi,

I'm trying to mark packets incoming from a given IP address, without success:

 modify VOIP_IN {
     rule 10 {
         action modify
         modify {
             mark 1
         }
         protocol udp
         source {
             address <src-ip-address>/32
         }
     }
 }

The firewall statistic shows the rule accepts packages and doesn't mark any.  Why doesn't it mark any packages?