add check

src/Service/BookSuggestions.php

@@ -53,7 +53,7 @@ public function getCategorySuggestions(Book $book): array
                 return $suggestions;
             }
             foreach ($results['docs'] as $result) {
-                if(is_array($result['subject'])){
+                if(array_key_exists('subject',$result) && is_array($result['subject'])){
                     foreach ($result['subject'] as $category) {
                         $suggestions['tags'][$category] = $category;
                     }