diff --git a/includes/functions/print-issue.php b/includes/functions/print-issue.php
index 26c62e9..284da17 100644
--- a/includes/functions/print-issue.php
+++ b/includes/functions/print-issue.php
@@ -402,8 +402,11 @@ function get_side_metabox_order( $order ) {
 
 			// Make submit div last.
 			if ( 'side' === $location ) {
-				// Remove submitdiv by value.
-				unset( $order[ $location ][ array_flip( $order[ $location ] )['submitdiv'] ] );
+				$locations = array_flip( $order[ $location ] );
+				if ( isset( $locations['submitdiv'] ) ) {
+					unset( $order[ $location ][ $locations['submitdiv'] ] );
+				}
+
 				$order[ $location ][] = 'submitdiv';
 			}
 
diff --git a/readme.txt b/readme.txt
index eea247d..6433702 100644
--- a/readme.txt
+++ b/readme.txt
@@ -2,7 +2,7 @@
 Contributors:      10up, observerteam, joshlevinson, brs14ku, jeffpaul
 Tags:              print, workflow, editorial
 Requires at least: 5.7
-Tested up to:      6.3
+Tested up to:      6.4
 Stable tag:        1.2.3
 Requires PHP:      7.4
 License:           GPLv2 or later