diff --git a/moto/rds2/models.py b/moto/rds2/models.py
index 82349aa06..21155834b 100644
--- a/moto/rds2/models.py
+++ b/moto/rds2/models.py
@@ -524,7 +524,7 @@ class RDS2Backend(BaseBackend):
                                       }
         if option_group_kwargs['name'] in self.option_groups:
             raise RDSClientError('OptionGroupAlreadyExistsFault',
-                                 'An option group named {} already exists.'.format(option_group_kwargs['name']))
+                                 'An option group named {0} already exists.'.format(option_group_kwargs['name']))
         if 'description' not in option_group_kwargs or not option_group_kwargs['description']:
             raise RDSClientError('InvalidParameterValue',
                                  'The parameter OptionGroupDescription must be provided and must not be blank.')
@@ -545,7 +545,7 @@ class RDS2Backend(BaseBackend):
         if option_group_name in self.option_groups:
             return self.option_groups.pop(option_group_name)
         else:
-            raise RDSClientError('OptionGroupNotFoundFault', 'Specified OptionGroupName: {} not found.'.format(option_group_name))
+            raise RDSClientError('OptionGroupNotFoundFault', 'Specified OptionGroupName: {0} not found.'.format(option_group_name))
 
     def describe_option_groups(self, option_group_kwargs):
         option_group_list = []
@@ -575,7 +575,7 @@ class RDS2Backend(BaseBackend):
                 option_group_list.append(option_group)
         if not len(option_group_list):
             raise RDSClientError('OptionGroupNotFoundFault',
-                                 'Specified OptionGroupName: {} not found.'.format(option_group_kwargs['name']))
+                                 'Specified OptionGroupName: {0} not found.'.format(option_group_kwargs['name']))
         return option_group_list[marker:max_records+marker]
 
     @staticmethod
@@ -599,10 +599,10 @@ class RDS2Backend(BaseBackend):
             }
         }
         if engine_name not in default_option_group_options:
-            raise RDSClientError('InvalidParameterValue', 'Invalid DB engine: {}'.format(engine_name))
+            raise RDSClientError('InvalidParameterValue', 'Invalid DB engine: {0}'.format(engine_name))
         if major_engine_version and major_engine_version not in default_option_group_options[engine_name]:
             raise RDSClientError('InvalidParameterCombination',
-                                 'Cannot find major version {} for {}'.format(major_engine_version, engine_name))
+                                 'Cannot find major version {0} for {1}'.format(major_engine_version, engine_name))
         if major_engine_version:
             return default_option_group_options[engine_name][major_engine_version]
         return default_option_group_options[engine_name]['all']
@@ -610,7 +610,7 @@ class RDS2Backend(BaseBackend):
     def modify_option_group(self, option_group_name, options_to_include=None, options_to_remove=None, apply_immediately=None):
         if option_group_name not in self.option_groups:
             raise RDSClientError('OptionGroupNotFoundFault',
-                                 'Specified OptionGroupName: {} not found.'.format(option_group_name))
+                                 'Specified OptionGroupName: {0} not found.'.format(option_group_name))
         if not options_to_include and not options_to_remove:
             raise RDSClientError('InvalidParameterValue',
                                  'At least one option must be added, modified, or removed.')
@@ -651,7 +651,7 @@ class RDS2Backend(BaseBackend):
                     return self.subnet_groups[resource_name].get_tags()
         else:
             raise RDSClientError('InvalidParameterValue',
-                                 'Invalid resource name: {}'.format(arn))
+                                 'Invalid resource name: {0}'.format(arn))
         return []
 
     def remove_tags_from_resource(self, arn, tag_keys):