diff --git a/types/whatwg-streams/index.d.ts b/types/whatwg-streams/index.d.ts
index e2487d438d..cadcf346ca 100644
--- a/types/whatwg-streams/index.d.ts
+++ b/types/whatwg-streams/index.d.ts
@@ -71,7 +71,7 @@ declare class ReadableStreamBYOBReader<R = ArrayBufferView> {
 }
 
 declare class ReadableStreamDefaultController<R = ArrayBufferView> {
-    readonly desiredSize: number;
+    readonly desiredSize: number | null;
 
     close(): void;
     enqueue(chunk: R): void;
@@ -80,7 +80,7 @@ declare class ReadableStreamDefaultController<R = ArrayBufferView> {
 
 declare class ReadableByteStreamController<R = ArrayBufferView> {
     readonly byobRequest: ReadableStreamBYOBRequest<R>;
-    readonly desiredSize: number;
+    readonly desiredSize: number | null;
 
     close(): void;
     enqueue(chunk: R): void;
@@ -157,5 +157,5 @@ declare class TransformStreamDefaultController<R> {
     error(reason: any): void;
     terminate(): void;
 
-    readonly desiredSize: number;
+    readonly desiredSize: number | null;
 }
diff --git a/types/whatwg-streams/whatwg-streams-tests.ts b/types/whatwg-streams/whatwg-streams-tests.ts
index 56246e0d97..c6e83a89d3 100644
--- a/types/whatwg-streams/whatwg-streams-tests.ts
+++ b/types/whatwg-streams/whatwg-streams-tests.ts
@@ -48,7 +48,7 @@ function makeReadableBackpressureSocketStream(host: string, port: number) {
             socket.ondata = (event: any) => {
                 controller.enqueue(event.data);
 
-                if (controller.desiredSize <= 0) {
+                if (controller.desiredSize! <= 0) {
                     // The internal queue is full, so propagate
                     // the backpressure signal to the underlying source.
                     socket.readStop();