diff --git a/src/lib/readable-stream/byte-stream-controller.ts b/src/lib/readable-stream/byte-stream-controller.ts
index 3d11bbb..3a171ff 100644
--- a/src/lib/readable-stream/byte-stream-controller.ts
+++ b/src/lib/readable-stream/byte-stream-controller.ts
@@ -996,18 +996,12 @@ export function SetUpReadableByteStreamControllerFromUnderlyingSource(
   const controller: ReadableByteStreamController = Object.create(ReadableByteStreamController.prototype);
 
   let startAlgorithm: () => void | PromiseLike<void> = () => undefined;
-  let pullAlgorithm: () => Promise<void> = () => promiseResolvedWith(undefined);
-  let cancelAlgorithm: (reason: any) => Promise<void> = () => promiseResolvedWith(undefined);
+  let pullAlgorithm: () => Promise<void> = underlyingByteSource.pull !== undefined ? () => underlyingByteSource.pull!(controller) : () => promiseResolvedWith(undefined);
+  let cancelAlgorithm: (reason: any) => Promise<void> = underlyingByteSource.cancel !== undefined ? reason => underlyingByteSource.cancel!(reason) : () => promiseResolvedWith(undefined);
 
   if (underlyingByteSource.start !== undefined) {
     startAlgorithm = () => underlyingByteSource.start!(controller);
   }
-  if (underlyingByteSource.pull !== undefined) {
-    pullAlgorithm = () => underlyingByteSource.pull!(controller);
-  }
-  if (underlyingByteSource.cancel !== undefined) {
-    cancelAlgorithm = reason => underlyingByteSource.cancel!(reason);
-  }
 
   const autoAllocateChunkSize = underlyingByteSource.autoAllocateChunkSize;
   if (autoAllocateChunkSize === 0) {
diff --git a/src/lib/readable-stream/default-controller.ts b/src/lib/readable-stream/default-controller.ts
index 7f1d200..824cd27 100644
--- a/src/lib/readable-stream/default-controller.ts
+++ b/src/lib/readable-stream/default-controller.ts
@@ -375,18 +375,12 @@ export function SetUpReadableStreamDefaultControllerFromUnderlyingSource<R>(
   const controller: ReadableStreamDefaultController<R> = Object.create(ReadableStreamDefaultController.prototype);
 
   let startAlgorithm: () => void | PromiseLike<void> = () => undefined;
-  let pullAlgorithm: () => Promise<void> = () => promiseResolvedWith(undefined);
-  let cancelAlgorithm: (reason: any) => Promise<void> = () => promiseResolvedWith(undefined);
+  let pullAlgorithm: () => Promise<void> = underlyingSource.pull !== undefined ? () => underlyingSource.pull!(controller) : () => promiseResolvedWith(undefined);
+  let cancelAlgorithm: (reason: any) => Promise<void> = underlyingSource.cancel !== undefined ? reason => underlyingSource.cancel!(reason) : () => promiseResolvedWith(undefined);
 
   if (underlyingSource.start !== undefined) {
     startAlgorithm = () => underlyingSource.start!(controller);
   }
-  if (underlyingSource.pull !== undefined) {
-    pullAlgorithm = () => underlyingSource.pull!(controller);
-  }
-  if (underlyingSource.cancel !== undefined) {
-    cancelAlgorithm = reason => underlyingSource.cancel!(reason);
-  }
 
   SetUpReadableStreamDefaultController(
     stream, controller, startAlgorithm, pullAlgorithm, cancelAlgorithm, highWaterMark, sizeAlgorithm