diff --git a/lib/core/codec-worker.js b/lib/core/codec-worker.js
index bad4c8df..59f04ef7 100644
--- a/lib/core/codec-worker.js
+++ b/lib/core/codec-worker.js
@@ -100,9 +100,9 @@ class ProgressWatcherStream extends TransformStream {
 	constructor(readableSource, { onstart, onprogress, size, onend }) {
 		let chunkOffset = 0;
 		super({
-			start() {
+			async start() {
 				if (onstart) {
-					callHandler(onstart, size);
+					await callHandler(onstart, size);
 				}
 			},
 			async transform(chunk, controller) {
@@ -112,10 +112,10 @@ class ProgressWatcherStream extends TransformStream {
 				}
 				controller.enqueue(chunk);
 			},
-			flush() {
+			async flush() {
 				readableSource.size = chunkOffset;
 				if (onend) {
-					callHandler(onend, chunkOffset);
+					await callHandler(onend, chunkOffset);
 				}
 			}
 		});