diff --git a/src/main/java/org/ntlab/citrusserver/repositories/PublicBookManager.java b/src/main/java/org/ntlab/citrusserver/repositories/PublicBookManager.java
index 23e239e..fb9717b 100644
--- a/src/main/java/org/ntlab/citrusserver/repositories/PublicBookManager.java
+++ b/src/main/java/org/ntlab/citrusserver/repositories/PublicBookManager.java
@@ -106,7 +106,9 @@
             }
             this.publicBooks.get(account.getId()).add(book);
         } else{
-            this.publicBooks.get(account.getId()).remove(book);
+            if(publicBooks.containsKey(account.getId())){
+                this.publicBooks.get(account.getId()).remove(book);
+            }
         }
     }
 }