Check that the IP is set before adding a new line

lib/vagrant-hostmanager/hosts_file.rb

@@ -80,7 +80,9 @@ module VagrantPlugins
         ip = get_ip_address(machine, resolving_machine)
         host = machine.config.vm.hostname || machine.name
         aliases = machine.config.hostmanager.aliases.join(' ').chomp
-        "#{ip}\t#{host} #{aliases}\n"
+        if ip != nil
+          "#{ip}\t#{host} #{aliases}\n"
+        end
       end
 
       def get_ip_address(machine, resolving_machine)

test/Vagrantfile

@@ -33,4 +33,9 @@ Vagrant.configure('2') do |config|
     server.vm.network :private_network, :ip => '10.0.5.4'
     server.vm.provision :hostmanager
   end
-end
+
+  config.vm.define :server4 do |server|
+    server.vm.hostname = 'scruffy'
+    server.vm.provision :hostmanager
+  end
+end