diff --git a/src/isobmff/isobmff-demuxer.ts b/src/isobmff/isobmff-demuxer.ts
index 814dc8e..ddaaafa 100644
--- a/src/isobmff/isobmff-demuxer.ts
+++ b/src/isobmff/isobmff-demuxer.ts
@@ -3151,17 +3151,24 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 
 			data = readBytes(slice, sampleInfo.sampleSize);
 
-			if (this.internalTrack.encryptionAuxInfo) {
-				assert(this.internalTrack.encryptionInfo);
+			if (this.internalTrack.encryptionInfo) {
+				let sampleEncryption: SampleEncryptionInfo | null = null;
 
-				const entries = await resolveEncryptionAuxInfo(
-					this.internalTrack.demuxer.reader,
-					this.internalTrack.encryptionInfo,
-					this.internalTrack.encryptionAuxInfo,
-				);
+				if (this.internalTrack.encryptionAuxInfo) {
+					const entries = await resolveEncryptionAuxInfo(
+						this.internalTrack.demuxer.reader,
+						this.internalTrack.encryptionInfo,
+						this.internalTrack.encryptionAuxInfo,
+					);
 
-				if (sampleIndex < entries.length) {
-					data = await decryptSample(this.internalTrack, entries[sampleIndex]!, data, null);
+					if (sampleIndex < entries.length) {
+						sampleEncryption = entries[sampleIndex]!;
+					}
+				}
+
+				sampleEncryption ??= getDefaultSampleEncryption(this.internalTrack.encryptionInfo);
+				if (sampleEncryption) {
+					data = await decryptSample(this.internalTrack, sampleEncryption, data, null);
 				}
 			}
 		}
@@ -3207,8 +3214,12 @@ abstract class IsobmffTrackBacking implements InputTrackBacking {
 
 			data = readBytes(slice, fragmentSample.byteSize);
 
-			if (fragmentSample.encryption) {
-				data = await decryptSample(this.internalTrack, fragmentSample.encryption, data, fragment);
+			if (this.internalTrack.encryptionInfo) {
+				const sampleEncryption = fragmentSample.encryption
+					?? getDefaultSampleEncryption(this.internalTrack.encryptionInfo);
+				if (sampleEncryption) {
+					data = await decryptSample(this.internalTrack, sampleEncryption, data, fragment);
+				}
 			}
 		}
 
@@ -3779,6 +3790,17 @@ const resolveEncryptionAuxInfo = async (
 	return entries;
 };
 
+const getDefaultSampleEncryption = (encryptionInfo: TrackEncryptionInfo): SampleEncryptionInfo | null => {
+	if (!encryptionInfo.defaultConstantIv) {
+		return null;
+	}
+
+	return {
+		iv: encryptionInfo.defaultConstantIv,
+		subsamples: null,
+	};
+};
+
 const decryptSample = async (
 	track: InternalTrack,
 	sampleEncryption: SampleEncryptionInfo,
diff --git a/test/browser/read-mp4.test.ts b/test/browser/read-mp4.test.ts
new file mode 100644
index 0000000..de0aa4a
--- /dev/null
+++ b/test/browser/read-mp4.test.ts
@@ -0,0 +1,27 @@
+import { expect, test } from 'vitest';
+import { Input } from '../../src/input.js';
+import { UrlSource } from '../../src/source.js';
+import { ALL_FORMATS } from '../../src/input-format.js';
+import { AudioSampleSink } from '../../src/media-sink.js';
+import { assert } from '../../src/misc.js';
+
+test('Encrypted MP4 without senc', async () => {
+	using input = new Input({
+		source: new UrlSource('/639955605-b752f7ad-7ce8-43c2-8ae9-88e74c6ae696.mp4'),
+		formats: ALL_FORMATS,
+		formatOptions: {
+			isobmff: {
+				resolveKeyId: () => 'aabbccddeeff00112233445566778899',
+			},
+		},
+	});
+
+	const audioTrack = await input.getPrimaryAudioTrack();
+	assert(audioTrack);
+
+	// Test that it can decode
+	const sink = new AudioSampleSink(audioTrack);
+	using firstSample = await sink.getSample(0);
+	assert(firstSample);
+	expect(firstSample.timestamp).toBe(0);
+});
diff --git a/test/public/639955605-b752f7ad-7ce8-43c2-8ae9-88e74c6ae696.mp4 b/test/public/639955605-b752f7ad-7ce8-43c2-8ae9-88e74c6ae696.mp4
new file mode 100755
index 0000000..d5c3b2a
Binary files /dev/null and b/test/public/639955605-b752f7ad-7ce8-43c2-8ae9-88e74c6ae696.mp4 differ