diff --git a/dev/convert.html b/dev/convert.html
index 9f18bfd..658b6b4 100644
--- a/dev/convert.html
+++ b/dev/convert.html
@@ -189,7 +189,7 @@
 			},
 			trim: {
 				//start: 0,
-				//end: 10
+				end: 10
 			},
 		});
 		console.log(conversion);
diff --git a/dev/demux.html b/dev/demux.html
index baa5541..3b87013 100644
--- a/dev/demux.html
+++ b/dev/demux.html
@@ -21,7 +21,11 @@
 		const sink = new Mediabunny.EncodedPacketSink(videoTrack);
 
 		for await (const packet of sink.packets()) {
-			console.log(packet.timestamp);
+			console.log(packet.type, packet.timestamp);
+
+			if (packet.timestamp >= 5) {
+				break;
+			}
 		}
 
 		/*
diff --git a/src/muxer.ts b/src/muxer.ts
index ee0e7bd..83def00 100644
--- a/src/muxer.ts
+++ b/src/muxer.ts
@@ -46,12 +46,16 @@ export abstract class Muxer {
 
 	private trackTimestampInfo = new WeakMap<OutputTrack, {
 		maxTimestamp: number;
-		maxTimestampBeforeLastKeyPacket: number;
+		maxTimestampBeforeLastKeyPacket: number | null;
 	}>();
 
 	protected validateAndNormalizeTimestamp(track: OutputTrack, timestampInSeconds: number, isKeyPacket: boolean) {
 		timestampInSeconds += track.source._timestampOffset;
 
+		if (timestampInSeconds < 0) {
+			throw new Error(`Timestamps must be non-negative (got ${timestampInSeconds}s).`);
+		}
+
 		let timestampInfo = this.trackTimestampInfo.get(track);
 		if (!timestampInfo) {
 			if (!isKeyPacket) {
@@ -60,29 +64,28 @@ export abstract class Muxer {
 
 			timestampInfo = {
 				maxTimestamp: timestampInSeconds,
-				maxTimestampBeforeLastKeyPacket: timestampInSeconds,
+				maxTimestampBeforeLastKeyPacket: null,
 			};
 			this.trackTimestampInfo.set(track, timestampInfo);
-		}
+		} else {
+			if (isKeyPacket) {
+				timestampInfo.maxTimestampBeforeLastKeyPacket = timestampInfo.maxTimestamp;
+			}
 
-		if (timestampInSeconds < 0) {
-			throw new Error(`Timestamps must be non-negative (got ${timestampInSeconds}s).`);
-		}
+			if (
+				timestampInfo.maxTimestampBeforeLastKeyPacket !== null
+				&& timestampInSeconds < timestampInfo.maxTimestampBeforeLastKeyPacket
+			) {
+				throw new Error(
+					`Timestamps cannot be smaller than the largest timestamp of the previous GOP (a GOP begins with a`
+					+ ` key packet and ends right before the next key packet). Got ${timestampInSeconds}s, but largest`
+					+ ` timestamp is ${timestampInfo.maxTimestampBeforeLastKeyPacket}s.`,
+				);
+			}
 
-		if (isKeyPacket) {
-			timestampInfo.maxTimestampBeforeLastKeyPacket = timestampInfo.maxTimestamp;
+			timestampInfo.maxTimestamp = Math.max(timestampInfo.maxTimestamp, timestampInSeconds);
 		}
 
-		if (timestampInSeconds < timestampInfo.maxTimestampBeforeLastKeyPacket) {
-			throw new Error(
-				`Timestamps cannot be smaller than the largest timestamp of the previous GOP (a GOP begins with a key`
-				+ ` packet and ends right before the next key packet). Got ${timestampInSeconds}s, but largest`
-				+ ` timestamp is ${timestampInfo.maxTimestampBeforeLastKeyPacket}s.`,
-			);
-		}
-
-		timestampInfo.maxTimestamp = Math.max(timestampInfo.maxTimestamp, timestampInSeconds);
-
 		return timestampInSeconds;
 	}
 }