diff --git a/manifest.yaml b/manifest.yaml
index 9f08786..7363c1b 100644
--- a/manifest.yaml
+++ b/manifest.yaml
@@ -8,8 +8,8 @@ dublin_core:
     description: ""
     format: text/usfm
     identifier: ulb
-    issued: 2020-12-21
-    modified: 2020-12-21
+    issued: '2020-12-21'
+    modified: '2020-12-21'
     language:
         direction: ltr
         identifier: ru