Index: wp-admin/update-core.php
===================================================================
--- wp-admin/update-core.php	(revision 15908)
+++ wp-admin/update-core.php	(working copy)
@@ -394,6 +394,7 @@
 	wp_version_check();
 	require_once(ABSPATH . 'wp-admin/admin-header.php');
 	core_upgrade_preamble();
+	include(ABSPATH . 'wp-admin/admin-footer.php');
 } elseif ( 'do-core-upgrade' == $action || 'do-core-reinstall' == $action ) {
 	check_admin_referer('upgrade-core');
 
@@ -413,6 +414,7 @@
 	if ( isset( $_POST['upgrade'] ) )
 		do_core_upgrade($reinstall);
 
+	include(ABSPATH . 'wp-admin/admin-footer.php');
 } elseif ( 'do-plugin-upgrade' == $action ) {
 	check_admin_referer('upgrade-core');
 
@@ -436,6 +438,7 @@
 	echo '<h2>' . esc_html__('Update Plugins') . '</h2>';
 	echo "<iframe src='$url' style='width: 100%; height: 100%; min-height: 750px;' frameborder='0'></iframe>";
 	echo '</div>';
+	include(ABSPATH . 'wp-admin/admin-footer.php');
 } elseif ( 'do-theme-upgrade' == $action ) {
 	check_admin_referer('upgrade-core');
 
@@ -459,6 +462,8 @@
 	echo '<h2>' . esc_html__('Update Themes') . '</h2>';
 	echo "<iframe src='$url' style='width: 100%; height: 100%; min-height: 750px;' frameborder='0'></iframe>";
 	echo '</div>';
+	include(ABSPATH . 'wp-admin/admin-footer.php');
+} else {
+	do_action( 'do-custom-upgrade_' . $action );
 }
 
-include(ABSPATH . 'wp-admin/admin-footer.php');