Ticket #30967: 30967.3.diff

src/wp-includes/formatting.php

@@ -1600 +1600 @@
         //Limit to A-Z,a-z,0-9,_,-
         $sanitized = preg_replace( '/[^A-Za-z0-9_-]/', '', $sanitized );
 
-        if ( '' == $sanitized )
-                $sanitized = $fallback;
-
+        if ( '' == $sanitized && $fallback ) {
+                return sanitize_html_class( $fallback );
+        }
         /**
          * Filter a sanitized HTML class string.
          *