diff --git src/wp-includes/theme.php src/wp-includes/theme.php
index 7514dd9..13bf9ca 100644
--- src/wp-includes/theme.php
+++ src/wp-includes/theme.php
@@ -1824,7 +1824,7 @@ function get_editor_stylesheets() {
  */
 function get_theme_starter_content() {
 	$theme_support = get_theme_support( 'starter-content' );
-	if ( ! empty( $theme_support ) ) {
+	if ( is_array( $theme_support ) && ! empty( $theme_support[0] ) && is_array( $theme_support[0] ) ) {
 		$config = $theme_support[0];
 	} else {
 		$config = array();
diff --git tests/phpunit/tests/theme/getThemeStarterContent.php tests/phpunit/tests/theme/getThemeStarterContent.php
new file mode 100644
index 0000000..5201260
--- /dev/null
+++ tests/phpunit/tests/theme/getThemeStarterContent.php
@@ -0,0 +1,180 @@
+<?php
+
+/**
+ * Tests get_theme_starter_content().
+ *
+ * @group themes
+ */
+class Tests_WP_Theme_Get_Theme_Starter_Content extends WP_UnitTestCase {
+
+	/**
+	 * Testing passing an empty array as starter content.
+	 */
+	function test_add_theme_support_empty() {
+		add_theme_support( 'starter-content', array() );
+		$starter_content = get_theme_starter_content();
+
+		$this->assertEmpty( $starter_content );
+	}
+
+	/**
+	 * Testing passing nothing as starter content.
+	 */
+	function test_add_theme_support_single_param() {
+		add_theme_support( 'starter-content' );
+		$starter_content = get_theme_starter_content();
+
+		$this->assertEmpty( $starter_content );
+	}
+
+	/**
+	 * Testing that placeholder starter content gets expanded, that unrecognized placeholders are discarded, and that custom items are recognized.
+	 */
+	function test_default_content_sections() {
+
+		/*
+		 * All placeholder identifiers should be referenced in this sample starter
+		 * content and then tested to ensure they get hydrated in the call to
+		 * get_theme_starter_content() to ensure that the starter content
+		 * placeholder identifiers remain intact in core.
+		 */
+		$dehydrated_starter_content = array(
+			'widgets' => array(
+				'sidebar-1' => array(
+					'text_business_info',
+					'text_about',
+					'archives',
+					'calendar',
+					'categories',
+					'meta',
+					'recent-comments',
+					'recent-posts',
+					'search',
+					'unknown',
+					'meta_custom' => array( 'meta', array(
+						'title' => 'Pre-hydrated meta widget.'
+					) ),
+				),
+			),
+			'nav_menus' => array(
+				'top' => array(
+					'name'  => 'Menu Name',
+					'items' => array(
+						'page_home',
+						'page_about',
+						'page_blog',
+						'page_news',
+						'page_contact',
+						'link_email',
+						'link_facebook',
+						'link_foursquare',
+						'link_github',
+						'link_instagram',
+						'link_linkedin',
+						'link_pinterest',
+						'link_twitter',
+						'link_yelp',
+						'link_youtube',
+						'link_unknown',
+						'link_custom' => array(
+							'title' => 'Custom',
+							'url' => 'https://custom.example.com/'
+						),
+					),
+				),
+			),
+			'posts' => array(
+				'home',
+				'about',
+				'contact',
+				'blog',
+				'news',
+				'homepage-section',
+				'unknown',
+				'custom' => array(
+					'post_type' => 'post',
+					'post_title' => 'Custom',
+				),
+			),
+			'options' => array(
+				'show_on_front'  => 'page',
+				'page_on_front'  => '{{home}}',
+				'page_for_posts' => '{{blog}}',
+			),
+			'theme_mods' => array(
+				'panel_1' => '{{homepage-section}}',
+				'panel_2' => '{{about-us}}',
+				'panel_3' => '{{blog}}',
+				'panel_4' => '{{contact-us}}',
+			),
+		);
+
+		add_theme_support( 'starter-content', $dehydrated_starter_content );
+
+		$hydrated_starter_content = get_theme_starter_content();
+		$this->assertSame( $hydrated_starter_content['theme_mods'], $dehydrated_starter_content['theme_mods'] );
+		$this->assertSame( $hydrated_starter_content['options'], $dehydrated_starter_content['options'] );
+		$this->assertCount( 16, $hydrated_starter_content['nav_menus']['top']['items'], 'Unknown should be dropped, custom should be present.' );
+		$this->assertCount( 10, $hydrated_starter_content['widgets']['sidebar-1'], 'Unknown should be dropped.' );
+
+		foreach ( $hydrated_starter_content['widgets']['sidebar-1'] as $widget ) {
+			$this->assertInternalType( 'array', $widget );
+			$this->assertCount( 2, $widget );
+			$this->assertInternalType( 'string', $widget[0] );
+			$this->assertInternalType( 'array', $widget[1] );
+			$this->assertArrayHasKey( 'title', $widget[1] );
+		}
+
+		foreach ( $hydrated_starter_content['nav_menus']['top']['items'] as $nav_menu_item ) {
+			$this->assertInternalType( 'array', $nav_menu_item );
+			$this->assertTrue( ! empty( $nav_menu_item['object_id'] ) || ! empty( $nav_menu_item['url'] ) );
+		}
+
+		foreach ( $hydrated_starter_content['posts'] as $key => $post ) {
+			$this->assertInternalType( 'string', $key );
+			$this->assertFalse( is_numeric( $key ) );
+			$this->assertInternalType( 'array', $post );
+			$this->assertArrayHasKey( 'post_type', $post );
+			$this->assertArrayHasKey( 'post_title', $post );
+		}
+	}
+
+	/**
+	 * Testing the filter with the text_credits widget.
+	 */
+	function test_get_theme_starter_content_filter() {
+
+		add_theme_support( 'starter-content',
+			array(
+				'widgets' => array(
+					'sidebar-1' => array(
+						'text_about',
+					),
+				),
+			)
+		);
+
+		add_filter( 'get_theme_starter_content', array( $this, 'filter_theme_starter_content' ), 10, 2 );
+		$starter_content = get_theme_starter_content();
+
+		$this->assertCount( 2, $starter_content['widgets']['sidebar-1'] );
+		$this->assertEquals( 'Filtered Widget', $starter_content['widgets']['sidebar-1'][1][1]['title'] );
+	}
+
+	/**
+	 * Filter the append a widget starter content.
+	 *
+	 * @param array $content Starter content (hydrated).
+	 * @param array $config  Starter content config (pre-hydrated).
+	 * @return array Filtered starter content.
+	 */
+	public function filter_theme_starter_content( $content, $config ) {
+		$this->assertInternalType( 'array', $config );
+		$this->assertCount( 1, $config['widgets']['sidebar-1'] );
+		$content['widgets']['sidebar-1'][] = array( 'text', array(
+			'title' => 'Filtered Widget',
+			'text'  => 'Custom ',
+		) );
+		return $content;
+	}
+}