Ticket #43374: 43374.2.diff

src/wp-includes/theme.php

@@ -369 +369 @@
  */
 function get_theme_roots() {
         global $wp_theme_directories;
-
-        if ( count( $wp_theme_directories ) <= 1 ) {
+        
+        // count() does not check the value before performin so we have to verify if empty first.
+        if ( empty( $wp_theme_directories ) || count( $wp_theme_directories ) <= 1 ) {
                 return '/themes';
         }