diff --git a/python/zensical/config.py b/python/zensical/config.py
index 884a729..c1e5604 100644
--- a/python/zensical/config.py
+++ b/python/zensical/config.py
@@ -448,6 +448,8 @@ def _apply_defaults(config: dict, path: str) -> dict:
     for fence in superfences.get("custom_fences", []):
         if isinstance(fence.get("format"), str):
             fence["format"] = _resolve(fence.get("format"))
+            if isinstance(fence.get("validator"), str):
+                fence["validator"] = _resolve(fence.get("validator"))
 
     # Ensure the table of contents title is initialized, as it's used inside
     # the template, and the table of contents extension is always defined