diff --git a/src/Service/BlockClassHelperService.php b/src/Service/BlockClassHelperService.php
index bac9bb0..f8b51f2 100644
--- a/src/Service/BlockClassHelperService.php
+++ b/src/Service/BlockClassHelperService.php
@@ -199,6 +199,8 @@ class BlockClassHelperService {
       $current_block_classes = explode(' ', $block_classes ?? '');

       // Merge with the current one.
+      $block_classes_stored = is_array($block_classes_stored) ? $block_classes_stored : [];
+      $current_block_classes = is_array($current_block_classes) ? $current_block_classes : [];
       $block_classes_to_store = array_merge($block_classes_stored, $current_block_classes);
       $block_classes_to_store = array_unique($block_classes_to_store);