diff --git a/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java b/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
index 0b4e3eb..0d2b459 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/BookManager.java
@@ -137,6 +137,9 @@
     }
 
     public void changeAccountId(String oldAccountId, String newAccountId) {
+        if(!booksMap.containsKey(oldAccountId)) {
+            return;
+        }
         for(Book book : booksMap.get(oldAccountId).values()) {
             book.setAccountId(newAccountId);
         }