diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d9ba102c..f1cf6ada 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -64,6 +64,10 @@ jobs:
               run: npm install
             - name: Check Types
               run: npm run test:types
+            - name: Check Types (TypeScript 5.x)
+              run: npm run test:types:5.x
+            - name: Check Types (TypeScript 5.3)
+              run: npm run test:types:5.3
     are_the_types_wrong:
         name: Are the types wrong?
         runs-on: ubuntu-latest
diff --git a/package.json b/package.json
index 6e091e3a..abd51aa2 100644
--- a/package.json
+++ b/package.json
@@ -57,7 +57,10 @@
     "test": "mocha \"tests/**/*.test.js\"",
     "test:coverage": "c8 npm test",
     "test:jsr": "npx -y jsr@latest publish --dry-run",
-    "test:types": "npm run build && tsc -p tests/types/tsconfig.json"
+    "test:types": "npm run build && tsc -p tests/types/tsconfig.json",
+    "test:types:5.x": "npx -p typescript@5.x -y -- tsc -p tests/types/tsconfig.json",
+    "test:types:5.3": "npx -p typescript@5.3 -y -- tsc -p tests/types/tsconfig.json",
+    "test:types:all": "npm run test:types && npm run test:types:5.x && npm run test:types:5.3"
   },
   "keywords": [
     "eslint",
@@ -89,7 +92,7 @@
     "mocha": "^11.0.0",
     "prettier": "3.8.2",
     "tailwind-csstree": "^0.3.0",
-    "typescript": "^5.9.3",
+    "typescript": "^6.0.3",
     "web-features": "^3.23.1",
     "yorkie": "^2.0.0"
   },
diff --git a/src/index.js b/src/index.js
index 23dac7c5..5624a513 100644
--- a/src/index.js
+++ b/src/index.js
@@ -34,10 +34,7 @@ const plugin = {
 	},
 };
 
-// eslint-disable-next-line no-lone-blocks -- The block syntax { ... } ensures that TypeScript does not get confused about the type of `plugin`.
-{
-	plugin.configs.recommended.plugins.css = plugin;
-}
+Object.assign(plugin.configs.recommended.plugins, { css: plugin });
 
 export default plugin;
 export { CSSSourceCode };
diff --git a/src/util.js b/src/util.js
index d87ba59d..df8b8b5f 100644
--- a/src/util.js
+++ b/src/util.js
@@ -17,18 +17,18 @@
 
 /**
  * Determines if an error is a syntax match error.
- * @param {Object} error The error object to check.
+ * @param {SyntaxMatchError | SyntaxReferenceError | Error} error The error object to check.
  * @returns {error is SyntaxMatchError} True if the error is a syntax match error, false if not.
  */
 export function isSyntaxMatchError(error) {
-	return typeof error.syntax === "string";
+	return "syntax" in error && typeof error.syntax === "string";
 }
 
 /**
  * Determines if an error is a syntax reference error.
- * @param {Object} error The error object to check.
+ * @param {SyntaxMatchError | SyntaxReferenceError | Error} error The error object to check.
  * @returns {error is SyntaxReferenceError} True if the error is a syntax reference error, false if not.
  */
 export function isSyntaxReferenceError(error) {
-	return typeof error.reference === "string";
+	return "reference" in error && typeof error.reference === "string";
 }
diff --git a/tests/types/tsconfig.json b/tests/types/tsconfig.json
index 4d6f9c30..f669877d 100644
--- a/tests/types/tsconfig.json
+++ b/tests/types/tsconfig.json
@@ -1,9 +1,14 @@
 {
   "extends": "../../tsconfig.json",
   "compilerOptions": {
+    "allowJs": false,
+    "checkJs": false,
     "noEmit": true,
     "rootDir": "../..",
     "strict": true,
+    "strictNullChecks": true,
+    "useUnknownInCatchVariables": true,
+    "noImplicitAny": true,
     "exactOptionalPropertyTypes": true
   },
   "files": [],
diff --git a/tsconfig.json b/tsconfig.json
index 25070aa8..bbb7a9bf 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -10,6 +10,10 @@
     "lib": ["ES2022"],
     "moduleResolution": "NodeNext",
     "module": "NodeNext",
+    "rootDir": "./src",
+    "strictNullChecks": false,
+    "useUnknownInCatchVariables": false,
+    "noImplicitAny": false,
     "types": []
   }
 }