diff --git a/src/wp-includes/theme.php b/src/wp-includes/theme.php
index 2a2922950d..4e4edd838e 100644
--- a/src/wp-includes/theme.php
+++ b/src/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';
 	}