diff --git a/src/lib/byte-length-queuing-strategy.ts b/src/lib/byte-length-queuing-strategy.ts
index 091bd6b..494b59d 100644
--- a/src/lib/byte-length-queuing-strategy.ts
+++ b/src/lib/byte-length-queuing-strategy.ts
@@ -69,5 +69,5 @@ export function IsByteLengthQueuingStrategy(x: any): x is ByteLengthQueuingStrat
     return false;
   }
 
-  return true;
+  return x instanceof ByteLengthQueuingStrategy;
 }
diff --git a/src/lib/count-queuing-strategy.ts b/src/lib/count-queuing-strategy.ts
index 0c20974..f0d219f 100644
--- a/src/lib/count-queuing-strategy.ts
+++ b/src/lib/count-queuing-strategy.ts
@@ -70,5 +70,5 @@ export function IsCountQueuingStrategy(x: any): x is CountQueuingStrategy {
     return false;
   }
 
-  return true;
+  return x instanceof CountQueuingStrategy;
 }
diff --git a/src/lib/readable-stream.ts b/src/lib/readable-stream.ts
index 1ff79fc..fc2015a 100644
--- a/src/lib/readable-stream.ts
+++ b/src/lib/readable-stream.ts
@@ -413,7 +413,7 @@ export function IsReadableStream(x: unknown): x is ReadableStream {
     return false;
   }
 
-  return true;
+  return x instanceof ReadableStream;
 }
 
 export function IsReadableStreamDisturbed(stream: ReadableStream): boolean {
diff --git a/src/lib/readable-stream/async-iterator.ts b/src/lib/readable-stream/async-iterator.ts
index 7433590..1197d7f 100644
--- a/src/lib/readable-stream/async-iterator.ts
+++ b/src/lib/readable-stream/async-iterator.ts
@@ -173,7 +173,12 @@ function IsReadableStreamAsyncIterator<R = any>(x: any): x is ReadableStreamAsyn
     return false;
   }
 
-  return true;
+  try {
+    // noinspection SuspiciousTypeOfGuard
+    return (x as ReadableStreamAsyncIteratorInstance<any>)._asyncIteratorImpl instanceof ReadableStreamAsyncIteratorImpl;
+  } catch {
+    return false;
+  }
 }
 
 // Helper functions for the ReadableStream.
diff --git a/src/lib/readable-stream/byob-reader.ts b/src/lib/readable-stream/byob-reader.ts
index 99b6b86..7836fd3 100644
--- a/src/lib/readable-stream/byob-reader.ts
+++ b/src/lib/readable-stream/byob-reader.ts
@@ -237,7 +237,7 @@ export function IsReadableStreamBYOBReader(x: any): x is ReadableStreamBYOBReade
     return false;
   }
 
-  return true;
+  return x instanceof ReadableStreamBYOBReader;
 }
 
 function ReadableStreamBYOBReaderRead<T extends ArrayBufferView>(reader: ReadableStreamBYOBReader,
diff --git a/src/lib/readable-stream/byte-stream-controller.ts b/src/lib/readable-stream/byte-stream-controller.ts
index f4ad44d..e35b205 100644
--- a/src/lib/readable-stream/byte-stream-controller.ts
+++ b/src/lib/readable-stream/byte-stream-controller.ts
@@ -388,7 +388,7 @@ export function IsReadableByteStreamController(x: any): x is ReadableByteStreamC
     return false;
   }
 
-  return true;
+  return x instanceof ReadableByteStreamController;
 }
 
 function IsReadableStreamBYOBRequest(x: any): x is ReadableStreamBYOBRequest {
@@ -400,7 +400,7 @@ function IsReadableStreamBYOBRequest(x: any): x is ReadableStreamBYOBRequest {
     return false;
   }
 
-  return true;
+  return x instanceof ReadableStreamBYOBRequest;
 }
 
 function ReadableByteStreamControllerCallPullIfNeeded(controller: ReadableByteStreamController): void {
diff --git a/src/lib/readable-stream/default-controller.ts b/src/lib/readable-stream/default-controller.ts
index 0db4999..1908d16 100644
--- a/src/lib/readable-stream/default-controller.ts
+++ b/src/lib/readable-stream/default-controller.ts
@@ -156,7 +156,7 @@ function IsReadableStreamDefaultController<R = any>(x: any): x is ReadableStream
     return false;
   }
 
-  return true;
+  return x instanceof ReadableStreamDefaultController;
 }
 
 function ReadableStreamDefaultControllerCallPullIfNeeded(controller: ReadableStreamDefaultController<any>): void {
diff --git a/src/lib/readable-stream/default-reader.ts b/src/lib/readable-stream/default-reader.ts
index 7791fc1..6237982 100644
--- a/src/lib/readable-stream/default-reader.ts
+++ b/src/lib/readable-stream/default-reader.ts
@@ -219,7 +219,7 @@ export function IsReadableStreamDefaultReader<R = any>(x: any): x is ReadableStr
     return false;
   }
 
-  return true;
+  return x instanceof ReadableStreamDefaultReader;
 }
 
 export function ReadableStreamDefaultReaderRead<R>(reader: ReadableStreamDefaultReader<R>,
diff --git a/src/lib/transform-stream.ts b/src/lib/transform-stream.ts
index 7b1c444..be056b5 100644
--- a/src/lib/transform-stream.ts
+++ b/src/lib/transform-stream.ts
@@ -220,7 +220,7 @@ function IsTransformStream(x: unknown): x is TransformStream {
     return false;
   }
 
-  return true;
+  return x instanceof TransformStream;
 }
 
 // This is a no-op if both sides are already errored.
@@ -348,7 +348,7 @@ function IsTransformStreamDefaultController<O = any>(x: any): x is TransformStre
     return false;
   }
 
-  return true;
+  return x instanceof TransformStreamDefaultController;
 }
 
 function SetUpTransformStreamDefaultController<I, O>(stream: TransformStream<I, O>,
diff --git a/src/lib/writable-stream.ts b/src/lib/writable-stream.ts
index 8b591de..f681506 100644
--- a/src/lib/writable-stream.ts
+++ b/src/lib/writable-stream.ts
@@ -277,7 +277,7 @@ function IsWritableStream(x: unknown): x is WritableStream {
     return false;
   }
 
-  return true;
+  return x instanceof WritableStream;
 }
 
 function IsWritableStreamLocked(stream: WritableStream): boolean {
@@ -786,7 +786,7 @@ function IsWritableStreamDefaultWriter<W = any>(x: any): x is WritableStreamDefa
     return false;
   }
 
-  return true;
+  return x instanceof WritableStreamDefaultWriter;
 }
 
 // A client of WritableStreamDefaultWriter may use these functions directly to bypass state check.
@@ -997,7 +997,7 @@ function IsWritableStreamDefaultController(x: any): x is WritableStreamDefaultCo
     return false;
   }
 
-  return true;
+  return x instanceof WritableStreamDefaultController;
 }
 
 function SetUpWritableStreamDefaultController<W>(stream: WritableStream<W>,