diff --git a/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java b/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java
index 18913c6..4eb354f 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/ScheduleManager.java
@@ -49,8 +49,9 @@
     public Schedule addSchedule(String accountId, int year, int month, int day, String title, String startTime, String endTime, Integer bookId){
         if(bookId == null || bookId.equals(0)){
             bookId = 0;
-        }
-        else if(!bookManager.getBooks(accountId).containsKey(bookId))return null;
+        } else if (bookManager.getBooks(accountId)==null) {
+            return null;
+        } else if(!bookManager.getBooks(accountId).containsKey(bookId))return null;
 
         if(!schedules.containsKey(accountId)){
             schedules.put(accountId, new HashMap<>());