diff --git a/src/frame/lib/frontmatter.ts b/src/frame/lib/frontmatter.ts
index bd8267a44d38..3beca783efd6 100644
--- a/src/frame/lib/frontmatter.ts
+++ b/src/frame/lib/frontmatter.ts
@@ -353,6 +353,9 @@ export const schema: Schema = {
     // START category-landing tags
     category: {
       type: 'array',
+      errorMessage: `must be an array, which is written in frontmatter like:
+category:
+  - Category Name`,
     },
     complexity: {
       type: 'array',