diff --git a/docs/NODE_API.md b/docs/NODE_API.md
index 6a1cbe0ad..26a85d7a0 100644
--- a/docs/NODE_API.md
+++ b/docs/NODE_API.md
@@ -105,7 +105,7 @@ Formats documentation as HTML.
 **Parameters**
 
 -   `comments` **[Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)<[Comment](https://developer.mozilla.org/en-US/docs/Web/API/Comment/Comment)>** parsed comments
--   `config` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output
+-   `config` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output (optional, default `{}`)
     -   `config.theme` **[string](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String)** Name of a module used for an HTML theme. (optional, default `'default_theme'`)
 
 **Examples**
@@ -132,7 +132,7 @@ Formats documentation as
 **Parameters**
 
 -   `comments` **[Array](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array)<[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)>** parsed comments
--   `args` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output
+-   `args` **[Object](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object)** Options that can customize the output (optional, default `{}`)
 
 **Examples**
 
diff --git a/src/output/html.js b/src/output/html.js
index 16a5f6f81..d7a4e0614 100644
--- a/src/output/html.js
+++ b/src/output/html.js
@@ -23,8 +23,8 @@ var mergeConfig = require('../merge_config');
  *     streamArray(output).pipe(vfs.dest('./output-directory'));
  *   });
  */
-function html(comments: Array<Comment>, config: DocumentationConfig) {
-  return mergeConfig(config).then(config => {
+function html(comments: Array<Comment>, config: Object = {}) {
+  return mergeConfig(config).then((config: DocumentationConfig) => {
     var themePath = '../default_theme/';
     if (config.theme) {
       themePath = path.resolve(process.cwd(), config.theme);
diff --git a/src/output/markdown.js b/src/output/markdown.js
index 29a3127e2..f496be07f 100644
--- a/src/output/markdown.js
+++ b/src/output/markdown.js
@@ -1,6 +1,7 @@
 /* @flow */
 
-var remark = require('remark'), markdownAST = require('./markdown_ast');
+var remark = require('remark'),
+  markdownAST = require('./markdown_ast');
 
 /**
  * Formats documentation as
@@ -22,7 +23,10 @@ var remark = require('remark'), markdownAST = require('./markdown_ast');
  *     fs.writeFileSync('./output.md', output);
  *   });
  */
-function markdown(comments: Array<Comment>, args: Object): Promise<string> {
+function markdown(
+  comments: Array<Comment>,
+  args: Object = {}
+): Promise<string> {
   return markdownAST(comments, args).then(ast => remark().stringify(ast));
 }