Index: wp-includes/theme.php
===================================================================
--- wp-includes/theme.php	(revision 21059)
+++ wp-includes/theme.php	(working copy)
@@ -98,7 +98,7 @@
 	if ( empty( $theme_root ) ) {
 		$theme_root = get_raw_theme_root( $stylesheet );
 		if ( false === $theme_root )
-			$theme_root = WP_CONTENT_DIR . $theme_root;
+			$theme_root = WP_CONTENT_DIR . '/themes';
 		elseif ( ! in_array( $theme_root, (array) $wp_theme_directories ) )
 			$theme_root = WP_CONTENT_DIR . $theme_root;
 	}