diff --git a/dev/convert.html b/dev/convert.html
index 36a2645..3778784 100644
--- a/dev/convert.html
+++ b/dev/convert.html
@@ -100,6 +100,7 @@
 			},
 			*/
 			video: () => ({
+				forceTranscode: true,
 				allowRotationMetadata: false,
 				//width: 720,
 				//frameRate: 30,
@@ -181,7 +182,7 @@
 			},
 			trim: {
 				start: 0,
-				end: 4
+				//end: 4
 			},
 		});
 		console.log(conversion);
diff --git a/docs/guide/converting-media-files.md b/docs/guide/converting-media-files.md
index 43d812a..8e11193 100644
--- a/docs/guide/converting-media-files.md
+++ b/docs/guide/converting-media-files.md
@@ -108,7 +108,7 @@ Sometimes, you may want to cancel an ongoing conversion process. For this, use t
 await conversion.cancel(); // Resolves once the conversion is canceled
 ```
 
-This automatically frees up all resources used by the conversion process.
+This automatically frees up all resources used by the conversion process and will cause any ongoing call to `execute` to throw a `ConversionCanceledError`.
 
 ## Video options
 
diff --git a/package-lock.json b/package-lock.json
index 6c803d8..c56b8e8 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "mediabunny",
-  "version": "1.27.4",
+  "version": "1.27.5",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "mediabunny",
-      "version": "1.27.4",
+      "version": "1.27.5",
       "license": "MPL-2.0",
       "workspaces": [
         "packages/*"
@@ -7739,9 +7739,9 @@
       }
     },
     "node_modules/mediabunny": {
-      "version": "1.27.3",
-      "resolved": "https://registry.npmjs.org/mediabunny/-/mediabunny-1.27.3.tgz",
-      "integrity": "sha512-hlzmgzMznp9DhA5fMJKS5yEAyfCUMxAc+DbSPxD4J1J2cYVl1L+pZLndkt5xLlD5aB5eHEnphHMW14ammMlUXg==",
+      "version": "1.27.4",
+      "resolved": "https://registry.npmjs.org/mediabunny/-/mediabunny-1.27.4.tgz",
+      "integrity": "sha512-j69K0yiXYMHyb5g4XyhYjbz1VpllRVyyDGF5lqYTsrdF0XoDRoGReoRvon9lnENv5yY5A5u+09dnqISwXvS86Q==",
       "license": "MPL-2.0",
       "peer": true,
       "workspaces": [
@@ -12065,7 +12065,7 @@
     },
     "packages/mp3-encoder": {
       "name": "@mediabunny/mp3-encoder",
-      "version": "1.27.4",
+      "version": "1.27.5",
       "license": "MPL-2.0",
       "devDependencies": {
         "@types/emscripten": "^1.40.1"
diff --git a/package.json b/package.json
index 94043f6..9b055b1 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "mediabunny",
   "author": "Vanilagy",
-  "version": "1.27.4",
+  "version": "1.27.5",
   "description": "Pure TypeScript media toolkit for reading, writing, and converting media files, directly in the browser.",
   "type": "module",
   "workspaces": [
diff --git a/packages/mp3-encoder/package.json b/packages/mp3-encoder/package.json
index cc1be31..9f82bf0 100644
--- a/packages/mp3-encoder/package.json
+++ b/packages/mp3-encoder/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@mediabunny/mp3-encoder",
   "author": "Vanilagy",
-  "version": "1.27.4",
+  "version": "1.27.5",
   "description": "MP3 encoder extension for Mediabunny, based on LAME.",
   "main": "./dist/bundles/mediabunny-mp3-encoder.mjs",
   "module": "./dist/bundles/mediabunny-mp3-encoder.mjs",
diff --git a/src/conversion.ts b/src/conversion.ts
index 862b6f0..a723d7b 100644
--- a/src/conversion.ts
+++ b/src/conversion.ts
@@ -822,7 +822,7 @@ export class Conversion {
 		}
 
 		if (this._canceled) {
-			await new Promise(() => {}); // Never resolve
+			throw new ConversionCanceledError();
 		}
 
 		await this.output.finalize();
@@ -832,7 +832,10 @@ export class Conversion {
 		}
 	}
 
-	/** Cancels the conversion process. Does nothing if the conversion is already complete. */
+	/**
+	 * Cancels the conversion process, causing any ongoing `execute` call to throw a `ConversionCanceledError`.
+	 * Does nothing if the conversion is already complete.
+	 */
 	async cancel() {
 		if (this.output.state === 'finalizing' || this.output.state === 'finalized') {
 			return;
@@ -1159,6 +1162,7 @@ export class Conversion {
 
 					for await (const sample of sink.samples(this._startTimestamp, this._endTimestamp)) {
 						if (this._canceled) {
+							sample.close();
 							lastSample?.close();
 							return;
 						}
@@ -1441,6 +1445,7 @@ export class Conversion {
 					const sink = new AudioSampleSink(track);
 					for await (const sample of sink.samples(undefined, this._endTimestamp)) {
 						if (this._canceled) {
+							sample.close();
 							return;
 						}
 
@@ -1551,6 +1556,7 @@ export class Conversion {
 
 			for await (const sample of iterator) {
 				if (this._canceled) {
+					sample.close();
 					return;
 				}
 
@@ -1589,6 +1595,19 @@ export class Conversion {
 	}
 }
 
+/**
+ * Thrown when a conversion couldn't complete due to being canceled.
+ * @group Conversion
+ * @public
+ */
+export class ConversionCanceledError extends Error {
+	/** Creates a new {@link ConversionCanceledError}. */
+	constructor(message = 'Conversion has been canceled.') {
+		super(message);
+		this.name = 'ConversionCanceledError';
+	}
+}
+
 const MAX_TIMESTAMP_GAP = 5;
 
 /**
diff --git a/src/index.ts b/src/index.ts
index 6cd723f..4c5bee1 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -190,6 +190,7 @@ export {
 	ConversionOptions,
 	ConversionVideoOptions,
 	ConversionAudioOptions,
+	ConversionCanceledError,
 	DiscardedTrack,
 } from './conversion';
 export {
diff --git a/src/input-format.ts b/src/input-format.ts
index 1748f27..db1e5b7 100644
--- a/src/input-format.ts
+++ b/src/input-format.ts
@@ -155,7 +155,7 @@ export class MatroskaInputFormat extends InputFormat {
 		}
 
 		const dataSize = readElementSize(headerSlice);
-		if (dataSize === null) {
+		if (typeof dataSize !== 'number') {
 			return false; // Miss me with that shit
 		}
 
@@ -171,7 +171,7 @@ export class MatroskaInputFormat extends InputFormat {
 
 			const { id, size } = header;
 			const dataStartPos = dataSlice.filePos;
-			if (size === null) return false;
+			if (size === undefined) return false;
 
 			switch (id) {
 				case EBMLId.EBMLVersion: {
diff --git a/src/matroska/ebml.ts b/src/matroska/ebml.ts
index eb27c40..891adab 100644
--- a/src/matroska/ebml.ts
+++ b/src/matroska/ebml.ts
@@ -7,7 +7,7 @@
  */
 
 import { MediaCodec } from '../codec';
-import { assertNever, textDecoder, textEncoder } from '../misc';
+import { assert, assertNever, textDecoder, textEncoder } from '../misc';
 import { FileSlice, readBytes, Reader, readF32Be, readF64Be, readU8 } from '../reader';
 import { Writer } from '../writer';
 
@@ -470,6 +470,10 @@ export const MIN_HEADER_SIZE = 2; // 1-byte ID and 1-byte size
 export const MAX_HEADER_SIZE = 2 * MAX_VAR_INT_SIZE; // 8-byte ID and 8-byte size
 
 export const readVarIntSize = (slice: FileSlice) => {
+	if (slice.remainingLength < 1) {
+		return null;
+	}
+
 	const firstByte = readU8(slice);
 	slice.skip(-1);
 
@@ -484,10 +488,19 @@ export const readVarIntSize = (slice: FileSlice) => {
 		mask >>= 1;
 	}
 
+	// Check if we have enough bytes to read the full varint
+	if (slice.remainingLength < width) {
+		return null;
+	}
+
 	return width;
 };
 
 export const readVarInt = (slice: FileSlice) => {
+	if (slice.remainingLength < 1) {
+		return null;
+	}
+
 	// Read the first byte to determine the width of the variable-length integer
 	const firstByte = readU8(slice);
 
@@ -503,6 +516,11 @@ export const readVarInt = (slice: FileSlice) => {
 		mask >>= 1;
 	}
 
+	if (slice.remainingLength < width - 1) {
+		// Not enough bytes
+		return null;
+	}
+
 	// First byte's value needs the marker bit cleared
 	let value = firstByte & (mask - 1);
 
@@ -563,39 +581,58 @@ export const readElementId = (slice: FileSlice) => {
 		return null;
 	}
 
+	if (slice.remainingLength < size) {
+		return null; // It don't fit
+	}
+
 	const id = readUnsignedInt(slice, size);
 	return id;
 };
 
-export const readElementSize = (slice: FileSlice) => {
-	let size: number | null = readU8(slice);
+/** Returns `undefined` to indicate the EBML undefined size. Returns `null` if the size couldn't be read. */
+export const readElementSize = (slice: FileSlice): number | undefined | null => {
+	// Need at least 1 byte to read the size
+	if (slice.remainingLength < 1) {
+		return null;
+	}
 
-	if (size === 0xff) {
-		size = null;
-	} else {
-		slice.skip(-1);
-		size = readVarInt(slice);
+	const firstByte = readU8(slice);
 
-		// In some (livestreamed) files, this is the value of the size field. While this technically is just a very
-		// large number, it is intended to behave like the reserved size 0xFF, meaning the size is undefined. We
-		// catch the number here. Note that it cannot be perfectly represented as a double, but the comparison works
-		// nonetheless.
-		// eslint-disable-next-line no-loss-of-precision
-		if (size === 0x00ffffffffffffff) {
-			size = null;
-		}
+	if (firstByte === 0xff) {
+		return undefined;
+	}
+
+	slice.skip(-1);
+	const size = readVarInt(slice);
+
+	if (size === null) {
+		return null;
+	}
+
+	// In some (livestreamed) files, this is the value of the size field. While this technically is just a very
+	// large number, it is intended to behave like the reserved size 0xFF, meaning the size is undefined. We
+	// catch the number here. Note that it cannot be perfectly represented as a double, but the comparison works
+	// nonetheless.
+	// eslint-disable-next-line no-loss-of-precision
+	if (size === 0x00ffffffffffffff) {
+		return undefined;
 	}
 
 	return size;
 };
 
 export const readElementHeader = (slice: FileSlice) => {
+	assert(slice.remainingLength >= MIN_HEADER_SIZE);
+
 	const id = readElementId(slice);
 	if (id === null) {
 		return null;
 	}
 
 	const size = readElementSize(slice);
+	if (size === null) {
+		return null;
+	}
 
 	return { id, size };
 };
@@ -720,8 +757,8 @@ export const CODEC_STRING_MAP: Partial<Record<MediaCodec, string>> = {
 	'webvtt': 'S_TEXT/WEBVTT',
 };
 
-export function assertDefinedSize(size: number | null): asserts size is number {
-	if (size === null) {
+export function assertDefinedSize(size: number | undefined): asserts size is number {
+	if (size === undefined) {
 		throw new Error('Undefined element size is used in a place where it is not supported.');
 	}
 };
diff --git a/src/matroska/matroska-demuxer.ts b/src/matroska/matroska-demuxer.ts
index db421b6..b373ff4 100644
--- a/src/matroska/matroska-demuxer.ts
+++ b/src/matroska/matroska-demuxer.ts
@@ -193,6 +193,7 @@ type InternalTrack = {
 	codecId: string | null;
 	codecPrivate: Uint8Array | null;
 	defaultDuration: number | null;
+	defaultDurationNs: number | null;
 	name: string | null;
 	languageCode: string;
 	decodingInstructions: DecodingInstruction[];
@@ -346,7 +347,7 @@ export class MatroskaDemuxer extends Demuxer {
 				} else if (id === EBMLId.Segment) { // Segment found!
 					await this.readSegment(dataStartPos, size);
 
-					if (size === null) {
+					if (size === undefined) {
 						// Segment sizes can be undefined (common in livestreamed files), so assume this is the last
 						// and only segment
 						break;
@@ -364,7 +365,7 @@ export class MatroskaDemuxer extends Demuxer {
 					// doesn't contain any of the clusters that follow it. In the case, we apply the following logic: if
 					// we find a top-level cluster, attribute it to the previous segment.
 
-					if (size === null) {
+					if (size === undefined) {
 						// Just in case this is one of those weird sizeless clusters, let's do our best and still try to
 						// determine its size.
 						const nextElementPos = await searchForNextElementId(
@@ -389,7 +390,7 @@ export class MatroskaDemuxer extends Demuxer {
 		})();
 	}
 
-	async readSegment(segmentDataStart: number, dataSize: number | null) {
+	async readSegment(segmentDataStart: number, dataSize: number | undefined) {
 		this.currentSegment = {
 			seekHeadSeen: false,
 			infoSeen: false,
@@ -406,7 +407,7 @@ export class MatroskaDemuxer extends Demuxer {
 			cuePoints: [],
 
 			dataStartPos: segmentDataStart,
-			elementEndPos: dataSize === null
+			elementEndPos: dataSize === undefined
 				? null // Assume it goes until the end of the file
 				: segmentDataStart + dataSize,
 			clusterSeekStartPos: segmentDataStart,
@@ -483,7 +484,7 @@ export class MatroskaDemuxer extends Demuxer {
 				break; // Stop at the first cluster
 			}
 
-			if (size === null) {
+			if (size === undefined) {
 				break;
 			} else {
 				currentPos = dataStartPos + size;
@@ -536,6 +537,13 @@ export class MatroskaDemuxer extends Demuxer {
 			this.currentSegment.timestampFactor = 1e9 / 1e6;
 		}
 
+		// Compute default duration for all tracks now that we have the timestamp factor
+		for (const track of this.currentSegment.tracks) {
+			if (track.defaultDurationNs !== null) {
+				track.defaultDuration = (this.currentSegment.timestampFactor * track.defaultDurationNs) / 1e9;
+			}
+		}
+
 		// Put default tracks first
 		this.currentSegment.tracks.sort((a, b) => Number(b.disposition.default) - Number(a.disposition.default));
 
@@ -606,7 +614,7 @@ export class MatroskaDemuxer extends Demuxer {
 		let size = elementHeader.size;
 		const dataStartPos = headerSlice.filePos;
 
-		if (size === null) {
+		if (size === undefined) {
 			// The cluster's size is undefined (can happen in livestreamed files). We'd still like to know the size of
 			// it, so we have no other choice but to iterate over the EBML structure until we find an element at level
 			// 0 or 1, indicating the end of the cluster (all elements inside the cluster are at level 2).
@@ -908,9 +916,7 @@ export class MatroskaDemuxer extends Demuxer {
 	}
 
 	readContiguousElements(slice: FileSlice, stopIds?: number[]) {
-		const startIndex = slice.filePos;
-
-		while (slice.filePos - startIndex <= slice.length - MIN_HEADER_SIZE) {
+		while (slice.remainingLength >= MIN_HEADER_SIZE) {
 			const startPos = slice.filePos;
 			const foundElement = this.traverseElement(slice, stopIds);
 
@@ -996,6 +1002,7 @@ export class MatroskaDemuxer extends Demuxer {
 					codecId: null,
 					codecPrivate: null,
 					defaultDuration: null,
+					defaultDurationNs: null,
 					name: null,
 					languageCode: UNDETERMINED_LANGUAGE,
 					decodingInstructions: [],
@@ -1005,6 +1012,11 @@ export class MatroskaDemuxer extends Demuxer {
 
 				this.readContiguousElements(slice.slice(dataStartPos, size));
 
+				// Check if track was disabled during parsing (e.g., by FlagEnabled being 0)
+				if (!this.currentTrack) {
+					break;
+				}
+
 				if (this.currentTrack.decodingInstructions.some((instruction) => {
 					return instruction.data?.type !== 'decompress'
 						|| instruction.scope !== ContentEncodingScope.Block
@@ -1203,9 +1215,7 @@ export class MatroskaDemuxer extends Demuxer {
 
 			case EBMLId.DefaultDuration: {
 				if (!this.currentTrack) break;
-
-				this.currentTrack.defaultDuration
-					= this.currentTrack.segment.timestampFactor * readUnsignedInt(slice, size) / 1e9;
+				this.currentTrack.defaultDurationNs = readUnsignedInt(slice, size);
 			}; break;
 
 			case EBMLId.Name: {
@@ -2222,7 +2232,7 @@ abstract class MatroskaTrackBacking implements InputTrackBacking {
 				}
 			}
 
-			if (size === null) {
+			if (size === undefined) {
 				// Undefined element size (can happen in livestreamed files). In this case, we need to do some
 				// searching to determine the actual size of the element.
 
diff --git a/src/media-source.ts b/src/media-source.ts
index 7b5cae6..ddf746b 100644
--- a/src/media-source.ts
+++ b/src/media-source.ts
@@ -134,12 +134,10 @@ export abstract class MediaSource {
 
 	/** @internal */
 	async _flushOrWaitForOngoingClose(forceClose: boolean) {
-		if (this._closingPromise) {
-			// Since closing also flushes, we don't want to do it twice
-			return this._closingPromise;
-		} else {
-			return this._flushAndClose(forceClose);
-		}
+		return this._closingPromise ??= (async () => {
+			await this._flushAndClose(forceClose);
+			this._closed = true;
+		})();
 	}
 }
 
diff --git a/test/browser/media-sources.test.ts b/test/browser/media-sources.test.ts
new file mode 100644
index 0000000..cd218ba
--- /dev/null
+++ b/test/browser/media-sources.test.ts
@@ -0,0 +1,35 @@
+import { test } from 'vitest';
+import { Output } from '../../src/output.js';
+import { WebMOutputFormat } from '../../src/output-format.js';
+import { BufferTarget } from '../../src/target.js';
+import { VideoSampleSource } from '../../src/media-source.js';
+import { VideoSample } from '../../src/sample.js';
+import { QUALITY_MEDIUM } from '../../src/encode.js';
+
+test('VideoSampleSource.close() should be idempotent after finalize()', async () => {
+	const output = new Output({
+		format: new WebMOutputFormat(),
+		target: new BufferTarget(),
+	});
+
+	const videoSource = new VideoSampleSource({
+		codec: 'vp8',
+		bitrate: QUALITY_MEDIUM,
+	});
+
+	output.addVideoTrack(videoSource);
+	await output.start();
+
+	const canvas = new OffscreenCanvas(100, 100);
+	const ctx = canvas.getContext('2d')!;
+	ctx.fillStyle = 'red';
+	ctx.fillRect(0, 0, 100, 100);
+
+	const sample = new VideoSample(canvas, { timestamp: 0, duration: 1 / 30 });
+	await videoSource.add(sample);
+	sample.close();
+
+	await output.finalize();
+
+	videoSource.close(); // This previously threw
+});