diff --git a/.github/workflows/prototyping.yml b/.github/workflows/prototyping.yml
index 5b5c53d4..fbe02362 100644
--- a/.github/workflows/prototyping.yml
+++ b/.github/workflows/prototyping.yml
@@ -8,7 +8,7 @@ on:
 
 env:
   AGDA_VERSION: 2.6.2.1
-  INDEX_STATE: '2022-03-03T21:49:20Z'
+  INDEX_STATE: !!str 2022-03-03T21:49:20Z
 
 jobs:
   linux: