diff --git a/src/input-format.ts b/src/input-format.ts
index 1c0e292..db1e5b7 100644
--- a/src/input-format.ts
+++ b/src/input-format.ts
@@ -154,11 +154,10 @@ export class MatroskaInputFormat extends InputFormat {
 			return false;
 		}
 
-		const dataSizeResult = readElementSize(headerSlice);
-		if (dataSizeResult === null || dataSizeResult.size === null) {
+		const dataSize = readElementSize(headerSlice);
+		if (typeof dataSize !== 'number') {
 			return false; // Miss me with that shit
 		}
-		const dataSize = dataSizeResult.size;
 
 		let dataSlice = input._reader.requestSlice(headerSlice.filePos, dataSize);
 		if (dataSlice instanceof Promise) dataSlice = await dataSlice;
@@ -172,7 +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 0df632d..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';
 
@@ -470,7 +470,6 @@ 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) => {
-	// Check if we have at least one byte to read
 	if (slice.remainingLength < 1) {
 		return null;
 	}
@@ -498,7 +497,6 @@ export const readVarIntSize = (slice: FileSlice) => {
 };
 
 export const readVarInt = (slice: FileSlice) => {
-	// Check if we have at least one byte
 	if (slice.remainingLength < 1) {
 		return null;
 	}
@@ -518,10 +516,8 @@ export const readVarInt = (slice: FileSlice) => {
 		mask >>= 1;
 	}
 
-	// Check if we have enough bytes remaining for the full varint
 	if (slice.remainingLength < width - 1) {
-		// Not enough bytes, rewind and return null
-		slice.skip(-1);
+		// Not enough bytes
 		return null;
 	}
 
@@ -585,19 +581,16 @@ export const readElementId = (slice: FileSlice) => {
 		return null;
 	}
 
+	if (slice.remainingLength < size) {
+		return null; // It don't fit
+	}
+
 	const id = readUnsignedInt(slice, size);
 	return id;
 };
 
-/**
- * Reads the size field of an EBML element.
- *
- * @returns
- * - `{ size: number }` - Successfully read a definite size
- * - `{ size: null }` - Successfully read an undefined size (0xFF marker, used in streaming)
- * - `null` - Couldn't read (insufficient bytes or invalid data)
- */
-export const readElementSize = (slice: FileSlice): { size: number | null } | null => {
+/** 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;
@@ -606,15 +599,13 @@ export const readElementSize = (slice: FileSlice): { size: number | null } | nul
 	const firstByte = readU8(slice);
 
 	if (firstByte === 0xff) {
-		// Legitimate undefined size marker
-		return { size: null };
+		return undefined;
 	}
 
 	slice.skip(-1);
 	const size = readVarInt(slice);
 
 	if (size === null) {
-		// Couldn't read the varint (insufficient bytes or invalid)
 		return null;
 	}
 
@@ -624,34 +615,26 @@ export const readElementSize = (slice: FileSlice): { size: number | null } | nul
 	// nonetheless.
 	// eslint-disable-next-line no-loss-of-precision
 	if (size === 0x00ffffffffffffff) {
-		return { size: null };
+		return undefined;
 	}
 
-	return { size };
+	return size;
 };
 
 export const readElementHeader = (slice: FileSlice) => {
-	// We need at least MIN_HEADER_SIZE bytes to read a header
-	if (slice.remainingLength < MIN_HEADER_SIZE) {
-		return null;
-	}
-
-	const startPos = slice.filePos;
+	assert(slice.remainingLength >= MIN_HEADER_SIZE);
 
 	const id = readElementId(slice);
 	if (id === null) {
-		slice.filePos = startPos;
 		return null;
 	}
 
-	const sizeResult = readElementSize(slice);
-	if (sizeResult === null) {
-		// Couldn't read size - rewind to start
-		slice.filePos = startPos;
+	const size = readElementSize(slice);
+	if (size === null) {
 		return null;
 	}
 
-	return { id, size: sizeResult.size };
+	return { id, size };
 };
 
 export const readAsciiString = (slice: FileSlice, length: number) => {
@@ -774,8 +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 ab2db28..feffbff 100644
--- a/src/matroska/matroska-demuxer.ts
+++ b/src/matroska/matroska-demuxer.ts
@@ -364,7 +364,7 @@ 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 +389,7 @@ 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 +406,7 @@ 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 +483,7 @@ export class MatroskaDemuxer extends Demuxer {
 				break; // Stop at the first cluster
 			}
 
-			if (size === null) {
+			if (size === undefined) {
 				break;
 			} else {
 				currentPos = dataStartPos + size;
@@ -606,7 +606,7 @@ 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).
@@ -2220,7 +2220,7 @@ 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.