diff --git a/src/things/book.ts b/src/things/book.ts
index 7d2505ab..86434db9 100644
--- a/src/things/book.ts
+++ b/src/things/book.ts
@@ -42,7 +42,7 @@ export interface SCBookWithoutReferences
    * @filterable
    * @keyword
    */
-  isbn: string;
+  isbn?: string;
 
   /**
    * Number of pages of a book