Index: src/wp-includes/canonical.php
===================================================================
--- src/wp-includes/canonical.php	(revision 28919)
+++ src/wp-includes/canonical.php	(working copy)
@@ -78,8 +78,8 @@
 	if ( !isset($redirect['query']) )
 		$redirect['query'] = '';
 
-	// If its not a preview then remove it from URL
-	if ( ! is_preview() ) {
+	// It's not a preview, so remove it from URL
+	if ( get_query_var( 'preview' ) ) {
 		$redirect['query'] = remove_query_arg( 'preview', $redirect['query'] );
 	}