I am not a Ruby expert, but it looks to me like the trouble is in /usr/lib/ruby/gems/1.8/gems/ohai-0.6.4/lib/ohai/plugins/ec2.rb. The ec2 plugin loops over a list of the host's network interfaces. It calls the network plugin to get that list, which is empty.
ec2.rb line 31:
network[:interfaces].values.each do |iface|
The loop never executes.