diff --git src/wp-includes/theme.php src/wp-includes/theme.php
index 3c908f1820..b1d85e6f94 100644
--- src/wp-includes/theme.php
+++ src/wp-includes/theme.php
@@ -35,7 +35,7 @@ function wp_get_themes( $args = array() ) {
 
 	$theme_directories = search_theme_directories();
 
-	if ( count( $wp_theme_directories ) > 1 ) {
+	if ( is_array( $wp_theme_directories ) && count( $wp_theme_directories ) > 1 ) {
 		// Make sure the current theme wins out, in case search_theme_directories() picks the wrong
 		// one in the case of a conflict. (Normally, last registered theme root wins.)
 		$current_theme = get_stylesheet();
@@ -627,8 +627,9 @@ function get_theme_root_uri( $stylesheet_or_template = false, $theme_root = fals
 function get_raw_theme_root( $stylesheet_or_template, $skip_cache = false ) {
 	global $wp_theme_directories;
 
-	if ( count($wp_theme_directories) <= 1 )
+	if ( ! is_array( $wp_theme_directories ) || count( $wp_theme_directories ) <= 1 ) {
 		return '/themes';
+	}
 
 	$theme_root = false;
 
diff --git tests/phpunit/tests/post/types.php tests/phpunit/tests/post/types.php
index 31a51c2020..3c1210e088 100644
--- tests/phpunit/tests/post/types.php
+++ tests/phpunit/tests/post/types.php
@@ -423,7 +423,8 @@ class Tests_Post_Types extends WP_UnitTestCase {
 			'public' => true,
 		) );
 
-		$this->assertSame( 1, count( $wp_filter['future_foo'] ) );
+		$this->assertArrayHasKey( 'future_foo', $wp_filter );
+		$this->assertSame( 1, count( $wp_filter['future_foo']->callbacks ) );
 		$this->assertTrue( unregister_post_type( 'foo' ) );
 		$this->assertArrayNotHasKey( 'future_foo', $wp_filter );
 	}
@@ -439,7 +440,8 @@ class Tests_Post_Types extends WP_UnitTestCase {
 			'register_meta_box_cb' => '__return_empty_string',
 		) );
 
-		$this->assertSame( 1, count( $wp_filter['add_meta_boxes_foo'] ) );
+		$this->assertArrayHasKey( 'add_meta_boxes_foo', $wp_filter );
+		$this->assertSame( 1, count( $wp_filter['add_meta_boxes_foo']->callbacks ) );
 		$this->assertTrue( unregister_post_type( 'foo' ) );
 		$this->assertArrayNotHasKey( 'add_meta_boxes_foo', $wp_filter );
 	}
diff --git tests/phpunit/tests/taxonomy.php tests/phpunit/tests/taxonomy.php
index 252280eef4..b5fd536100 100644
--- tests/phpunit/tests/taxonomy.php
+++ tests/phpunit/tests/taxonomy.php
@@ -764,7 +764,8 @@ class Tests_Taxonomy extends WP_UnitTestCase {
 
 		register_taxonomy( 'foo', 'post' );
 
-		$this->assertSame( 1, count( $wp_filter['wp_ajax_add-foo'] ) );
+		$this->assertArrayHasKey( 'wp_ajax_add-foo', $wp_filter );
+		$this->assertSame( 1, count( $wp_filter['wp_ajax_add-foo']->callbacks ) );
 		$this->assertTrue( unregister_taxonomy( 'foo' ) );
 		$this->assertArrayNotHasKey( 'wp_ajax_add-foo', $wp_filter );
 	}