diff --git a/quartz/plugins/transformers/ofm.ts b/quartz/plugins/transformers/ofm.ts
index 0e13cef9e..d49b8e670 100644
--- a/quartz/plugins/transformers/ofm.ts
+++ b/quartz/plugins/transformers/ofm.ts
@@ -820,6 +820,6 @@ declare module "vfile" {
   interface DataMap {
     blocks: Record<string, Element>
     htmlAst: HtmlRoot
-    hasMermaidDiagram: boolean
+    hasMermaidDiagram: boolean | undefined
   }
 }