multiple subnets through a single interface with dhcp relay

I have a pix in router mode and am trying to add a new /24 subnet. I added a VLAN for the new subnet and tagged it for trunking all the way to a single leaf node. unfortunately dhcp does not give it an address. the dhcp server log shows the request coming from the native interface on the pix instead of the subinterface. Do I need to put the old subnet on a subinterface with a VLAN and tag it for trunking as well? its vlan is untagged on all switches and not defined on the pix.