diff --git a/app/ece.js b/app/ece.js
index 631116a9..75cd3aa8 100644
--- a/app/ece.js
+++ b/app/ece.js
@@ -1,12 +1,8 @@
 import 'buffer';
-import TransformStream from './transformStream';
-import { ReadableStream as ReadableStreamPony } from 'web-streams-ponyfill';
-try {
-  new ReadableStream().pipeThrough(new TransformStream());
-} catch (e) {
-  // eslint-disable-next-line no-global-assign
-  ReadableStream = ReadableStreamPony;
-}
+import {
+  TStream as TransformStream,
+  RStream as ReadableStream
+} from './streams';
 
 const NONCE_LENGTH = 12;
 const TAG_LENGTH = 16;
@@ -360,14 +356,12 @@ export default class ECE {
         new BlobSlicer(this.input, this.rs, this.mode)
       );
     } else {
-      // eslint-disable-next-line no-undef
       const sliceStream = new TransformStream(
         new StreamSlicer(this.rs, this.mode)
       );
       inputStream = this.input.pipeThrough(sliceStream);
     }
 
-    // eslint-disable-next-line no-undef
     const cryptoStream = new TransformStream(
       new ECETransformer(this.mode, this.key, this.rs, this.salt)
     );
diff --git a/app/serviceWorker.js b/app/serviceWorker.js
index f017e433..f78b5cd3 100644
--- a/app/serviceWorker.js
+++ b/app/serviceWorker.js
@@ -1,6 +1,6 @@
 import Keychain from './keychain';
 import { downloadStream } from './api';
-import TransformStream from './transformStream';
+import { TStream as TransformStream, wrapReadable } from './streams';
 
 let noSave = false;
 const map = new Map();
@@ -30,7 +30,7 @@ async function decryptStream(request) {
       }
     });
 
-    const readStream = stream.pipeThrough(progStream);
+    const readStream = wrapReadable(stream).pipeThrough(progStream);
     const decrypted = file.keychain.decryptStream(readStream);
 
     const headers = {
@@ -38,7 +38,7 @@ async function decryptStream(request) {
       'Content-Type': file.type,
       'Content-Length': file.size
     };
-    const body = decrypted.local ? decrypted.nativeReadable : decrypted;
+    const body = decrypted.isPony ? decrypted.toNative() : decrypted;
     return new Response(body, { headers });
   } catch (e) {
     if (noSave) {
diff --git a/app/streams.js b/app/streams.js
new file mode 100644
index 00000000..5e8e254e
--- /dev/null
+++ b/app/streams.js
@@ -0,0 +1,35 @@
+/* global TransformStream ReadableStream */
+import { createReadableStreamWrapper } from '@mattiasbuelens/web-streams-adapter';
+import {
+  TransformStream as TransformStreamPony,
+  ReadableStream as ReadableStreamPony
+} from 'web-streams-ponyfill';
+
+const toNativeReadable = createReadableStreamWrapper(ReadableStream);
+const toPonyReadable = createReadableStreamWrapper(ReadableStreamPony);
+
+export let TStream;
+if (typeof TransformStream === 'function') {
+  TStream = TransformStream;
+} else {
+  TStream = TransformStreamPony;
+  TStream.prototype.isPony = true;
+}
+
+export let RStream = ReadableStream;
+try {
+  new ReadableStream().pipeThrough(new TransformStream());
+} catch (e) {
+  RStream = ReadableStreamPony;
+  RStream.prototype.isPony = true;
+  RStream.prototype.toNative = function() {
+    return toNativeReadable(this);
+  };
+}
+
+export function wrapReadable(stream) {
+  if (RStream === ReadableStream) {
+    return stream;
+  }
+  return toPonyReadable(stream);
+}
diff --git a/app/transformStream.js b/app/transformStream.js
deleted file mode 100644
index f0dc1605..00000000
--- a/app/transformStream.js
+++ /dev/null
@@ -1,25 +0,0 @@
-/* global TransformStream */
-import { createReadableStreamWrapper } from '@mattiasbuelens/web-streams-adapter';
-import { TransformStream as TransformStreamPony } from 'web-streams-ponyfill';
-
-const toNative = createReadableStreamWrapper(ReadableStream);
-
-class TransformStreamLocal {
-  constructor(transformer) {
-    this.stream = new TransformStreamPony(transformer);
-    this.local = true;
-  }
-  get nativeReadable() {
-    return toNative(this.stream.readable);
-  }
-  get readable() {
-    return this.stream.readable;
-  }
-  get writable() {
-    return this.stream.writable;
-  }
-}
-
-export default (typeof TransformStream === 'function'
-  ? TransformStream
-  : TransformStreamLocal);