diff --git a/includes/class-wp-push-syndication-server.php b/includes/class-wp-push-syndication-server.php
index 49f82f48b43be9baa5e581612ca3d3cafab68d31..a1f9b1954e4db531b298dab791e0c0ba5f5a54a9 100644
--- a/includes/class-wp-push-syndication-server.php
+++ b/includes/class-wp-push-syndication-server.php
@@ -1208,7 +1208,7 @@ class WP_Push_Syndication_Server {
 		}
 
 		// Order by last update date
-		usort( $sites, 'sort_sites_by_last_pull_date' );
+		usort( $sites, array( $this, 'sort_sites_by_last_pull_date' ) );
 
 		return $sites;
 	}