diff --git a/manifest.yaml b/manifest.yaml
index 4fe7a0b..cea8c03 100644
--- a/manifest.yaml
+++ b/manifest.yaml
@@ -7,8 +7,8 @@ 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