diff --git a/api/src/Entity/Book.php b/api/src/Entity/Book.php
index 5f359e8..bab1e88 100644
--- a/api/src/Entity/Book.php
+++ b/api/src/Entity/Book.php
@@ -27,9 +27,9 @@ class Book
     #[ORM\Id, ORM\Column, ORM\GeneratedValue]
      private ?int $id = null;
 
-     /** The ISBN of this book (or null if doesn't have one). */
-    #[ORM\Column(nullable: true)]
-     public ?string $isbn = null;
+     /** The ISBN of this book. */
+    #[ORM\Column]
+     public string $isbn = '';
 
      /** The title of this book. */
     #[ORM\Column]
diff --git a/docs/openapi.json b/docs/openapi.json
index 10295db..d459e13 100644
--- a/docs/openapi.json
+++ b/docs/openapi.json
@@ -794,8 +794,7 @@
                         "type": "integer"
                     },
                     "isbn": {
-                        "type": "string",
-                        "nullable": true
+                        "type": "string"
                     },
                     "title": {
                         "type": "string"
@@ -864,8 +863,7 @@
                         "type": "integer"
                     },
                     "isbn": {
-                        "type": "string",
-                        "nullable": true
+                        "type": "string"
                     },
                     "title": {
                         "type": "string"