diff --git a/src/index.ts b/src/index.ts
index f5c3d5c..9c1cd42 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -179,6 +179,16 @@ export default function ofm() {
                 }
 
                 tag = slugTag(tag)
+
+                if (!file.data)
+                    file.data = {};
+
+                if (!file.data.frontmatter)
+                    file.data.frontmatter = {};
+
+                if (!file.data.frontmatter.tags)
+                    file.data.frontmatter.tags = [];
+
                 if (file.data.frontmatter && !file.data.frontmatter.tags.includes(tag)) {
                     file.data.frontmatter.tags.push(tag)
                 }