diff --git a/test/prefix.test.js b/test/prefix.test.js
index df22a6f28..8fe76981c 100644
--- a/test/prefix.test.js
+++ b/test/prefix.test.js
@@ -4,3 +4,6 @@ const {prefix} = require('./_30s.js');
 test('prefix is a Function', () => {
   expect(prefix).toBeInstanceOf(Function);
 });
+test('prefix is a Function', () => {
+  expect(typeof prefix('appearance')).toBe('string');
+});