Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 42992)
+++ src/wp-includes/theme.php	(working copy)
@@ -370,7 +370,7 @@
 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';
 	}