diff --git a/index.js b/index.js
index cb6bd69..6fe0a8a 100644
--- a/index.js
+++ b/index.js
@@ -29,7 +29,8 @@ const fallbackDescriptorDuplicateCheck = allDescriptors => {
 		throw new Error('Only one fallback image candidate is allowed');
 	}
 
-	if (allDescriptors.x['1']) {
+	// TODO: Use `?`.
+	if (allDescriptors.x && allDescriptors.x['1']) {
 		throw new Error('A fallback image is equivalent to a 1x descriptor, providing both is invalid.');
 	}
 
diff --git a/test.js b/test.js
index 3cd1852..d86db6d 100644
--- a/test.js
+++ b/test.js
@@ -41,6 +41,21 @@ test('parseSrcset() should parse srcset separated without whitespaces', t => {
 	]);
 });
 
+test('parseSrcset() - strict mode', t => {
+	t.deepEqual(
+		parseSrcset('images/x.jpg, images/x-retina.jpg 2x', {strict: true}),
+		[
+			{
+				url: 'images/x.jpg',
+			},
+			{
+				url: 'images/x-retina.jpg',
+				density: 2,
+			},
+		],
+	);
+});
+
 test('stringifySrcset() should stringify srcset', t => {
 	const fixture = [
 		{url: 'banner-HD.jpeg', density: 2},