diff --git a/src/main/java/org/ntlab/citrusserver/resources/PublicBooksRest.java b/src/main/java/org/ntlab/citrusserver/resources/PublicBooksRest.java
index 99df9e9..71ce363 100644
--- a/src/main/java/org/ntlab/citrusserver/resources/PublicBooksRest.java
+++ b/src/main/java/org/ntlab/citrusserver/resources/PublicBooksRest.java
@@ -64,11 +64,11 @@
     @GET
     @Produces(MediaType.APPLICATION_JSON)
     public ArrayList<Book> searchBooksByTitleAndAccount(@QueryParam("search_title") String search_title, @QueryParam("search_account_id") String search_account_id) {
-        if(!search_title.isEmpty() && !search_account_id.isEmpty()) {
+        if(search_title != null && search_account_id != null) {
             return publicBookManager.searchBooksByTitleAndAccount(search_title, search_account_id);
-        } else if(!search_title.isEmpty()) {
+        } else if(search_title!= null) {
             return publicBookManager.searchBooksByTitle(search_title);
-        } else if(!search_account_id.isEmpty()) {
+        } else if(search_account_id != null) {
             return publicBookManager.searchBooksByAccount(search_account_id);
         } else {
             return publicBookManager.getAllPublicBooks();