diff --git a/blocks/tag_youtube/db/upgrade.php b/blocks/tag_youtube/db/upgrade.php
index 897eb1ebe14..fc0bf34f27c 100644
--- a/blocks/tag_youtube/db/upgrade.php
+++ b/blocks/tag_youtube/db/upgrade.php
@@ -101,6 +101,12 @@ function xmldb_block_tag_youtube_upgrade($oldversion) {
             // If the block uses a legacy category name, update it to use the current category ID instead.
             foreach ($blockinstances as $blockinstance) {
                 $blockconfig = unserialize(base64_decode($blockinstance->configdata));
+
+                // Skip if the block does not have a specific category set.
+                if (!isset($blockconfig->category)) {
+                    continue;
+                }
+
                 $blockcategoryconfig = $blockconfig->category;
                 // The block is using a legacy category name as a category config.
                 if (array_key_exists($blockcategoryconfig, $categorynamemap)) {