diff --git a/pre_commit/clientlib/validate_config.py b/pre_commit/clientlib/validate_config.py
index e4cbbddf..1da54f91 100644
--- a/pre_commit/clientlib/validate_config.py
+++ b/pre_commit/clientlib/validate_config.py
@@ -41,8 +41,8 @@ CONFIG_JSON_SCHEMA = {
                         },
                         'additional_dependencies': {
                             'type': 'array',
-                            'items': {'type': 'string'}
-                        }
+                            'items': {'type': 'string'},
+                        },
                     },
                     'required': ['id'],
                 }
diff --git a/pre_commit/clientlib/validate_manifest.py b/pre_commit/clientlib/validate_manifest.py
index c08ce0bf..e69e739f 100644
--- a/pre_commit/clientlib/validate_manifest.py
+++ b/pre_commit/clientlib/validate_manifest.py
@@ -38,6 +38,10 @@ MANIFEST_JSON_SCHEMA = {
                     'type': 'string',
                 },
             },
+            'additional_dependencies': {
+                'type': 'array',
+                'items': {'type': 'string'},
+            },
         },
         'required': ['id', 'name', 'entry', 'language', 'files'],
     },
diff --git a/pre_commit/repository.py b/pre_commit/repository.py
index 66374649..1d046911 100644
--- a/pre_commit/repository.py
+++ b/pre_commit/repository.py
@@ -55,7 +55,8 @@ class Repository(object):
         dep_dict = defaultdict(lambda: defaultdict(set))
         for _, hook in self.hooks:
             dep_dict[hook['language']][hook['language_version']].update(
-                hook.get('additional_dependencies', []))
+                hook.get('additional_dependencies', []),
+            )
         return dep_dict
 
     @cached_property
@@ -118,7 +119,8 @@ class Repository(object):
 
             language.install_environment(
                 self.cmd_runner, language_version,
-                self.additional_dependencies[language_name][language_version])
+                self.additional_dependencies[language_name][language_version],
+            )
             # Touch the .installed file (atomic) to indicate we've installed
             open(self.cmd_runner.path(directory, '.installed'), 'w').close()
 
diff --git a/tests/repository_test.py b/tests/repository_test.py
index 00415992..1872033e 100644
--- a/tests/repository_test.py
+++ b/tests/repository_test.py
@@ -294,7 +294,6 @@ def mock_repo_config():
         'hooks': [{
             'id': 'pyflakes',
             'files': '\\.py$',
-            'additional_dependencies': ['pep8']
         }],
     }
     config_wrapped = apply_defaults([config], CONFIG_JSON_SCHEMA)
@@ -345,12 +344,12 @@ def test_additional_python_dependencies_installed(tempdir_factory, store):
 def test_additional_ruby_dependencies_installed(tempdir_factory, store):
     path = make_repo(tempdir_factory, 'ruby_hooks_repo')
     config = make_config_from_repo(path)
-    config['hooks'][0]['additional_dependencies'] = ['mime-types']
+    config['hooks'][0]['additional_dependencies'] = ['thread_safe']
     repo = Repository.create(config, store)
     repo.run_hook(repo.hooks[0][1], [])
     with ruby.in_env(repo.cmd_runner, 'default') as env:
         output = env.run('gem list --local')[1]
-        assert 'mime-types' in output
+        assert 'thread_safe' in output
 
 
 @pytest.mark.integration