Index: wp-includes/post-template.php
--- wp-includes/post-template.php
+++ wp-includes/post-template.php
@@ -267,7 +267,9 @@
 		return __( 'There is no excerpt because this is a protected post.' );
 	}
 
-	return apply_filters( 'get_the_excerpt', $post->post_excerpt );
+	$post_excerpt = isset( $post->post_excerpt ) ? $post->post_excerpt : '';
+
+	return apply_filters( 'get_the_excerpt', $post_excerpt );
 }
 
 /**