Ticket #43374: 43374.patch

src/wp-includes/theme.php

@@ -370 +370 @@
 function get_theme_roots() {
         global $wp_theme_directories;
 
-        if ( count( $wp_theme_directories ) <= 1 ) {
+        if ( is_countable( $wp_theme_directories ) && count( $wp_theme_directories ) <= 1 ) {
                 return '/themes';
         }