diff --git a/src/isobmff/isobmff-muxer.ts b/src/isobmff/isobmff-muxer.ts
index c9baf86..bf3a89e 100644
--- a/src/isobmff/isobmff-muxer.ts
+++ b/src/isobmff/isobmff-muxer.ts
@@ -206,6 +206,8 @@ export class IsobmffMuxer extends Muxer {
 		this.isCmaf = format instanceof CmafOutputFormat;
 		this.minimumFragmentDuration = format._options.minimumFragmentDuration
 			?? (format instanceof CmafOutputFormat ? Infinity : 1);
+
+		this.auxWriter.start();
 	}
 
 	async start() {
diff --git a/test/node/webvtt.test.ts b/test/node/webvtt.test.ts
new file mode 100644
index 0000000..5f3f72c
--- /dev/null
+++ b/test/node/webvtt.test.ts
@@ -0,0 +1,57 @@
+import { test } from 'vitest';
+import { Output } from '../../src/output.js';
+import { MkvOutputFormat, Mp4OutputFormat } from '../../src/output-format.js';
+import { BufferTarget } from '../../src/target.js';
+import { TextSubtitleSource } from '../../src/media-source.js';
+
+test('ISOBMFF muxing', async () => {
+	const output = new Output({
+		format: new Mp4OutputFormat(),
+		target: new BufferTarget(),
+	});
+
+	const source = new TextSubtitleSource('webvtt');
+	output.addSubtitleTrack(source);
+
+	await output.start();
+
+	await source.add(`WEBVTT
+
+00:00.000 --> 00:00.900
+Hildy!
+
+00:01.000 --> 00:01.400
+How are you?
+
+00:01.500 --> 00:02.900
+Tell me, is the lord of the universe in?
+`);
+
+	await output.finalize();
+});
+
+test('Matroska muxing', async () => {
+	const output = new Output({
+		format: new MkvOutputFormat(),
+		target: new BufferTarget(),
+	});
+
+	const source = new TextSubtitleSource('webvtt');
+	output.addSubtitleTrack(source);
+
+	await output.start();
+
+	await source.add(`WEBVTT
+
+00:00.000 --> 00:00.900
+Hildy!
+
+00:01.000 --> 00:01.400
+How are you?
+
+00:01.500 --> 00:02.900
+Tell me, is the lord of the universe in?
+`);
+
+	await output.finalize();
+});