diff --git a/src/main/java/org/ntlab/citrusserver/repositories/TodoManager.java b/src/main/java/org/ntlab/citrusserver/repositories/TodoManager.java
index 069f791..4251341 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/TodoManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/TodoManager.java
@@ -102,6 +102,12 @@
      * @return そのアカウントの本に所属するtodoのうち、指定した年月日のtodoを返します
      */
     public HashMap<Integer, Todo> getTodosByDay(String accountId, int bookId, int year, int month, int day){
+        if(!todos.containsKey(accountId)){
+            return null;
+        }
+        if(!todos.get(accountId).containsKey((bookId))){
+            return null;
+        }
         if(!todos.get(accountId).get(bookId).containsKey(year)){
             return null;
         }