Index: src/wp-includes/class-wp.php
===================================================================
--- src/wp-includes/class-wp.php	(revision 35929)
+++ src/wp-includes/class-wp.php	(working copy)
@@ -609,7 +609,11 @@
 
 			$success = true;
 			if ( is_singular() ) {
-				$p = clone $wp_query->post;
+
+				$p = false;
+				if ( is_a( $wp_query->post, 'WP_Post' ) )
+					$p = clone $wp_query->post;
+
 				// Only set X-Pingback for single posts that allow pings.
 				if ( $p && pings_open( $p ) ) {
 					@header( 'X-Pingback: ' . get_bloginfo( 'pingback_url' ) );