When trying to add a new host with an ip address/mac that conflicts with an existing DHCP reservation, you get the following error message:

× Unable to save
DHCP Settings for zenith2.tuc.noao.edu task failed with the following error: Must define a hostname

entry from foreman/production.log

Processing HostsController#create (for 140.252.7.41 at 2011-11-22 12:47:41) [POST]
  Parameters: {"commit"=>"Submit", "action"=>"create", "authenticity_token"=>"R4K/F5qxuZXz3zdyzizvWMNKh+cvGzOpp9gn2ZmfUc8=", "controller"=>"hosts", "host"=>{"comment"=>"", "architecture_id"=>"1", "subnet_id"=>"1", "managed"=>"true", "puppetproxy_id"=>"1", "name"=>"zenith2.tuc.noao.edu", "sp_mac"=>"", "puppetmaster_name"=>"puppet.tuc.noao.edu", "sp_name"=>"", "mac"=>"00:15:17:ea:5c:e8", "domain_id"=>"1", "enabled"=>"1", "root_pass"=>"[FILTERED]", "build"=>"1", "medium_id"=>"5", "hostgroup_id"=>"2", "ptable_id"=>"12", "sp_subnet_id"=>"", "ip"=>"140.252.19.104", "environment_id"=>"1", "model_id"=>"", "disk"=>"", "sp_ip"=>"", "puppetclass_ids"=>[""], "operatingsystem_id"=>"1"}}
Create DHCP reservation for zenith2.tuc.noao.edu-00:15:17:ea:5c:e8/140.252.19.104
Conflicting DHCP reservation for zenith2.tuc.noao.edu-00:15:17:ea:5c:e8/140.252.19.104 detected
DHCP Settings for zenith2.tuc.noao.edu task failed with the following error: Must define a hostname
Rolling back due to a problem: DHCP Settings for zenith2.tuc.noao.edu     10     failed     zenith2.tuc.noao.eduset_dhcp
Failed to save: DHCP Settings for zenith2.tuc.noao.edu task failed with the following error: Must define a hostname
Rendering template within layouts/application
Rendering hosts/new
Completed in 488ms (View: 384, DB: 19) | 200 OK [https://puppet/hosts]