diff --git a/application/bookmark/BookmarkFileService.php b/application/bookmark/BookmarkFileService.php
index 804b2520..eb7899bf 100644
--- a/application/bookmark/BookmarkFileService.php
+++ b/application/bookmark/BookmarkFileService.php
@@ -349,7 +349,7 @@ class BookmarkFileService implements BookmarkServiceInterface
         $bookmarkDays = array_keys($bookmarkDays);
         sort($bookmarkDays);
 
-        return $bookmarkDays;
+        return array_map('strval', $bookmarkDays);
     }
 
     /**
diff --git a/tests/bookmark/BookmarkFileServiceTest.php b/tests/bookmark/BookmarkFileServiceTest.php
index 59c0608c..42485c99 100644
--- a/tests/bookmark/BookmarkFileServiceTest.php
+++ b/tests/bookmark/BookmarkFileServiceTest.php
@@ -690,12 +690,12 @@ class BookmarkFileServiceTest extends TestCase
      */
     public function testDays()
     {
-        $this->assertEquals(
+        $this->assertSame(
             ['20100309', '20100310', '20121206', '20121207', '20130614', '20150310'],
             $this->publicLinkDB->days()
         );
 
-        $this->assertEquals(
+        $this->assertSame(
             ['20100309', '20100310', '20121206', '20121207', '20130614', '20141125', '20150310'],
             $this->privateLinkDB->days()
         );