Fix get related order IDs method

changelog.txt

@@ -1,5 +1,8 @@
 *** WooCommerce Subscriptions Core Changelog ***
 
+= 7.2.0 - xxxx-xx-xx =
+* Fix - Regenerate subscriptions related order caches (renewal, resubscribe, switch) if it's stored as an invalid value to prevent fatal errors.
+
 = 7.1.2 - 2024-xx-xx =
 * Update - Show "FREE" instead of 0 when there is no shipping cost in the recurring totals section of the Cart and Checkout blocks (requires WooCommerce 9.0+).
 

includes/data-stores/class-wcs-related-order-store-cached-cpt.php

@@ -210,7 +210,9 @@ public function get_related_order_ids_from_cache( $subscription, $relation_type
 
 		$meta_data = $this->get_related_order_metadata( $subscription, $relation_type );
 
-		return $meta_data ? maybe_unserialize( $meta_data->meta_value ) : '';
+		$related_orders = $meta_data ? maybe_unserialize( $meta_data->meta_value ) : '';
+
+		return is_array( $related_orders ) ? $related_orders : '';
 	}
 
 	/**