diff --git a/index.js b/index.js
index ddb8b1c..629b868 100644
--- a/index.js
+++ b/index.js
@@ -8,7 +8,7 @@ export class TimeoutError extends Error {
 export default function pTimeout(promise, milliseconds, fallback, options) {
 	let timer;
 	const cancelablePromise = new Promise((resolve, reject) => {
-		if (typeof milliseconds !== 'number' || milliseconds < 0) {
+		if (typeof milliseconds !== 'number' || Math.sign(milliseconds) !== 1) {
 			throw new TypeError(`Expected \`milliseconds\` to be a positive number, got \`${milliseconds}\``);
 		}
 
diff --git a/test.js b/test.js
index ad86baf..c4d172b 100644
--- a/test.js
+++ b/test.js
@@ -20,6 +20,10 @@ test('throws when milliseconds is negative number', async t => {
 	await t.throwsAsync(pTimeout(delay(50), -1), {instanceOf: TypeError});
 });
 
+test('throws when milliseconds is NaN', async t => {
+	await t.throwsAsync(pTimeout(delay(50), Number.NaN), {instanceOf: TypeError});
+});
+
 test('handles milliseconds being `Infinity`', async t => {
 	t.is(
 		await pTimeout(delay(50, {value: fixture}), Number.POSITIVE_INFINITY),