diff --git a/lib/ansible/plugins/inventory/yaml.py b/lib/ansible/plugins/inventory/yaml.py
index d013fc4837..d8b9599365 100644
--- a/lib/ansible/plugins/inventory/yaml.py
+++ b/lib/ansible/plugins/inventory/yaml.py
@@ -45,7 +45,14 @@ all: # keys must be unique, i.e. only one 'hosts' per group
             children:
                 group_x:
                     hosts:
-                        test5
+                        test5   # Note that one machine will work without a colon
+                #group_x:
+                #    hosts:
+                #        test5  # But this won't
+                #        test7  #
+                group_y:
+                    hosts:
+                        test6:  # So always use a colon
             vars:
                 g2_var2: value3
             hosts: