diff --git a/index.js b/index.js
index 9748b9b..fa48cc7 100644
--- a/index.js
+++ b/index.js
@@ -9,7 +9,7 @@ class TimeoutError extends Error {
 }
 
 module.exports = (promise, ms, fallback) => new Promise((resolve, reject) => {
-	if (typeof ms !== 'number' && ms >= 0) {
+	if (typeof ms !== 'number' || ms < 0) {
 		throw new TypeError('Expected `ms` to be a positive number');
 	}
 
diff --git a/test.js b/test.js
index 7efba1a..68f3da3 100644
--- a/test.js
+++ b/test.js
@@ -9,6 +9,14 @@ test('resolves before timeout', async t => {
 	t.is(await m(delay(50).then(() => fixture), 200), fixture);
 });
 
+test('throws when ms is not number', async t => {
+	await t.throws(m(delay(50), '200'), TypeError);
+});
+
+test('throws when ms is negative number', async t => {
+	await t.throws(m(delay(50), -1), TypeError);
+});
+
 test('rejects after timeout', async t => {
 	await t.throws(m(delay(200), 50), m.TimeoutError);
 });