diff --git a/@vates/parse-duration/index.js b/@vates/parse-duration/index.js
index 66783a9def..4d534d93aa 100644
--- a/@vates/parse-duration/index.js
+++ b/@vates/parse-duration/index.js
@@ -6,9 +6,11 @@ exports.parseDuration = value => {
   if (typeof value === 'number') {
     return value
   }
-  const duration = ms(value)
-  if (duration === undefined) {
-    throw new TypeError(`not a valid duration: ${value}`)
+  if (typeof value === 'string' && value !== '') {
+    const duration = ms(value)
+    if (duration !== undefined) {
+      return duration
+    }
   }
-  return duration
+  throw new TypeError(`not a valid duration: ${value}`)
 }
diff --git a/@vates/parse-duration/index.test.mjs b/@vates/parse-duration/index.test.mjs
new file mode 100644
index 0000000000..dd873fd937
--- /dev/null
+++ b/@vates/parse-duration/index.test.mjs
@@ -0,0 +1,22 @@
+import { describe, it } from 'node:test'
+import { parseDuration } from '@vates/parse-duration'
+import assert from 'node:assert/strict'
+
+describe('parseDuration()', () => {
+  it('should parse string', () => {
+    const input = '2 days'
+    const expected = 172800000
+    assert.strictEqual(parseDuration(input), expected)
+  })
+
+  it('should return its input if already a number', () => {
+    const input = 172800000
+    assert.strictEqual(parseDuration(input), input)
+  })
+
+  for (const input of [undefined, '', 'invalid duration']) {
+    it('should throw an error for ' + input, () => {
+      assert.throws(() => parseDuration(input), { message: `not a valid duration: ${input}` })
+    })
+  }
+})
diff --git a/@vates/parse-duration/package.json b/@vates/parse-duration/package.json
index ebe0062a76..4dd15ead04 100644
--- a/@vates/parse-duration/package.json
+++ b/@vates/parse-duration/package.json
@@ -27,6 +27,7 @@
     "ms": "^2.1.2"
   },
   "scripts": {
-    "postversion": "npm publish --access public"
+    "postversion": "npm publish --access public",
+    "test": "node --test"
   }
 }