diff --git a/package.json b/package.json
index adeb9b2..77b7046 100644
--- a/package.json
+++ b/package.json
@@ -41,6 +41,7 @@
   "scripts": {
     "lint": "yarn eslint src/**/*.ts",
     "format": "prettier --write src/**/*.ts",
+    "format:check": "prettier --check src/**/*.ts",
     "prebuild": "rimraf dist",
     "build": "tsc && yarn run build:minified",
     "build:minified": "node scripts/build-minified.js",
diff --git a/src/utils.ts b/src/utils.ts
index 5641e9e..2de74be 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -59,7 +59,7 @@ export function createDocument(title: string, tags: string[] = []): Document {
   return {
     id: uuidv4(),
     title,
-    tags,
+    tags: tags.map((tag) => tag.toLowerCase()),
   };
 }