diff --git a/whatwg-streams/index.d.ts b/whatwg-streams/index.d.ts
index 7f171f99f6..45e33f40c3 100644
--- a/whatwg-streams/index.d.ts
+++ b/whatwg-streams/index.d.ts
@@ -15,6 +15,7 @@ interface ReadableByteStreamSource {
     cancel?(reason: string): void | Promise<void>;
 
     type: "bytes";
+    autoAllocateChunkSize?: number;
 }
 
 interface QueuingStrategy {