diff --git a/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java b/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
index 779fa8b..122e63a 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
@@ -26,6 +26,9 @@
     //本の新規作成
     public int createBook(String accountId, String title, String color, Boolean publicity)
     {
+        if(!booksMap.containsKey(accountId)){
+            booksMap.put(accountId, new HashMap<>());
+        }
         Account account = accountManager.getAccount(accountId); //アカウントの取得
         int newBookId = account.getNewBookId(); //新たに生成されたIdを取得（作成数もここで加算している）
         Book book = new Book(newBookId, title, publicity, color); //本の初期化