diff --git a/wp-includes/theme.php b/wp-includes/theme.php
index 2a2922950d..4e4edd838e 100644
--- a/wp-includes/theme.php
+++ b/wp-includes/theme.php
@@ -370,7 +370,7 @@ function get_template_directory_uri() {
 function get_theme_roots() {
 	global $wp_theme_directories;
 
-	if ( count( $wp_theme_directories ) <= 1 ) {
+	if ( ! is_array( $wp_theme_directories ) || count( $wp_theme_directories ) <= 1 ) {
 		return '/themes';
 	}
 
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 40bce2d998..b00bdc1447 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.0-alpha-43036';
+$wp_version = '5.0-alpha-43039';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.