Having issues getting live migrations to functions between two compute nodes.

One is an Opteron G4 and the other is an Opteron G5. I can migrate from the G4 to the G5 just fine. When I try to migrate an instance from the G5 node to the G4 node I get an error message:

InvalidCPUInfo: Unacceptable CPU info: CPU doesn't have compatibility.
This is despite having the following setting in my /etc/nova/nova.conf on the G5 machine.

[libvirt]
cpu_mode = custom
cpu_model = Opteron_G4

Is this not the setting I'm looking for? What can I do to make this work?