diff --git a/app/blobSlicer.js b/app/blobSlicer.js
new file mode 100644
index 00000000..a13d97a3
--- /dev/null
+++ b/app/blobSlicer.js
@@ -0,0 +1,27 @@
+export default class BlobSlicer {
+  constructor(blob, size) {
+    this.blob = blob;
+    this.index = 0;
+    this.chunkSize = size;
+  }
+
+  pull(controller) {
+    return new Promise((resolve, reject) => {
+      const bytesLeft = this.blob.size - this.index;
+      if (bytesLeft <= 0) {
+        controller.close();
+        return resolve();
+      }
+      const size = Math.min(this.chunkSize, bytesLeft);
+      const blob = this.blob.slice(this.index, this.index + size);
+      const reader = new FileReader();
+      reader.onload = () => {
+        controller.enqueue(new Uint8Array(reader.result));
+        resolve();
+      };
+      reader.onerror = reject;
+      reader.readAsArrayBuffer(blob);
+      this.index += size;
+    });
+  }
+}
diff --git a/app/ece.js b/app/ece.js
index 86ed29cd..952e0271 100644
--- a/app/ece.js
+++ b/app/ece.js
@@ -1,4 +1,5 @@
 import 'buffer';
+import BlobSlicer from './blobSlicer';
 import { transformStream } from './streams';
 
 const NONCE_LENGTH = 12;
@@ -225,40 +226,6 @@ class ECETransformer {
   }
 }
 
-export class BlobSlicer {
-  constructor(blob, rs, mode) {
-    this.blob = blob;
-    this.index = 0;
-    this.mode = mode;
-    this.chunkSize = mode === MODE_ENCRYPT ? rs - 17 : rs;
-  }
-
-  pull(controller) {
-    return new Promise((resolve, reject) => {
-      const bytesLeft = this.blob.size - this.index;
-      if (bytesLeft <= 0) {
-        controller.close();
-        return resolve();
-      }
-      let size = 1;
-      if (this.mode === MODE_DECRYPT && this.index === 0) {
-        size = Math.min(21, bytesLeft);
-      } else {
-        size = Math.min(this.chunkSize, bytesLeft);
-      }
-      const blob = this.blob.slice(this.index, this.index + size);
-      const reader = new FileReader();
-      reader.onload = () => {
-        controller.enqueue(new Uint8Array(reader.result));
-        resolve();
-      };
-      reader.onerror = reject;
-      reader.readAsArrayBuffer(blob);
-      this.index += size;
-    });
-  }
-}
-
 class StreamSlicer {
   constructor(rs, mode) {
     this.mode = mode;
@@ -350,7 +317,7 @@ export default class ECE {
 
     if (this.input instanceof Blob) {
       inputStream = new ReadableStream(
-        new BlobSlicer(this.input, this.rs, this.mode)
+        new BlobSlicer(this.input, this.rs - 17)
       );
     } else {
       inputStream = transformStream(
diff --git a/app/fileReceiver.js b/app/fileReceiver.js
index e694245b..d9ede78a 100644
--- a/app/fileReceiver.js
+++ b/app/fileReceiver.js
@@ -148,7 +148,7 @@ export default class FileReceiver extends Nanobus {
       this.state = 'complete';
     } catch (e) {
       this.downloadRequest = null;
-      if (e === 'cancelled') {
+      if (e === 'cancelled' || e.message === '400') {
         throw new Error(0);
       }
       throw e;
diff --git a/app/serviceWorker.js b/app/serviceWorker.js
index e2c5f271..ffe6be4c 100644
--- a/app/serviceWorker.js
+++ b/app/serviceWorker.js
@@ -15,8 +15,11 @@ self.addEventListener('activate', event => {
 });
 
 async function decryptStream(id) {
+  const file = map.get(id);
+  if (!file) {
+    return new Response(null, { status: 400 });
+  }
   try {
-    const file = map.get(id);
     const keychain = new Keychain(file.key, file.nonce);
     if (file.requiresPassword) {
       keychain.setPassword(file.password, file.url);
diff --git a/test/frontend/tests/streaming-tests.js b/test/frontend/tests/streaming-tests.js
index 44f898d8..2735f149 100644
--- a/test/frontend/tests/streaming-tests.js
+++ b/test/frontend/tests/streaming-tests.js
@@ -3,6 +3,7 @@ require('buffer');
 
 import assert from 'assert';
 import { b64ToArray } from '../../../app/utils';
+import BlobSlicer from '../../../app/blobSlicer';
 import ECE from '../../../app/ece.js';
 
 const rs = 36;
@@ -47,8 +48,10 @@ describe('Streaming', function() {
     });
 
     it('can decrypt', async function() {
-      const encBlob = new Blob([encrypted]);
-      const ece = new ECE(encBlob, key, 'decrypt', rs);
+      const blobStream = new ReadableStream(
+        new BlobSlicer(new Blob([encrypted]), rs)
+      );
+      const ece = new ECE(blobStream, key, 'decrypt', rs);
       const decStream = await ece.transform();
 
       const reader = decStream.getReader();
diff --git a/test/frontend/tests/workflow-tests.js b/test/frontend/tests/workflow-tests.js
index 0ed96108..70123820 100644
--- a/test/frontend/tests/workflow-tests.js
+++ b/test/frontend/tests/workflow-tests.js
@@ -153,26 +153,26 @@ describe('Upload / Download flow', function() {
     assert.equal(file.dtotal, 1);
   });
 
-  // it('does not increase download count when download cancelled', async function() {
-  //   const fs = new FileSender(blob);
-  //   const file = await fs.upload();
-  //   const fr = new FileReceiver({
-  //     secretKey: file.toJSON().secretKey,
-  //     id: file.id,
-  //     nonce: file.keychain.nonce,
-  //     requiresPassword: false
-  //   });
-  //   await fr.getMetadata();
-  //   fr.once('progress', () => fr.cancel());
+  it('does not increase download count when download cancelled', async function() {
+    const fs = new FileSender(blob);
+    const file = await fs.upload();
+    const fr = new FileReceiver({
+      secretKey: file.toJSON().secretKey,
+      id: file.id,
+      nonce: file.keychain.nonce,
+      requiresPassword: false
+    });
+    await fr.getMetadata();
+    fr.once('progress', () => fr.cancel());
 
-  //   try {
-  //     await fr.download(noSave);
-  //     assert.fail('not cancelled');
-  //   } catch (e) {
-  //     await file.updateDownloadCount();
-  //     assert.equal(file.dtotal, 0);
-  //   }
-  // });
+    try {
+      await fr.download(noSave);
+      assert.fail('not cancelled');
+    } catch (e) {
+      await file.updateDownloadCount();
+      assert.equal(file.dtotal, 0);
+    }
+  });
 
   it('can allow multiple downloads', async function() {
     const fs = new FileSender(blob);