diff --git a/tracker/tracker/package.json b/tracker/tracker/package.json
index 089f95f24..e5fdefddf 100644
--- a/tracker/tracker/package.json
+++ b/tracker/tracker/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@openreplay/tracker",
   "description": "The OpenReplay tracker main package",
-  "version": "3.5.6",
+  "version": "3.5.7",
   "keywords": [
     "logging",
     "replay"
diff --git a/tracker/tracker/src/webworker/QueueSender.ts b/tracker/tracker/src/webworker/QueueSender.ts
index b2cb4d3eb..d4686539d 100644
--- a/tracker/tracker/src/webworker/QueueSender.ts
+++ b/tracker/tracker/src/webworker/QueueSender.ts
@@ -1,5 +1,26 @@
 const INGEST_PATH = "/v1/web/i"
 
+const KEEPALIVE_SIZE_LIMIT = 64 << 10 // 64 kB
+
+// function sendXHR(url: string, token: string, batch: Uint8Array): Promise<XMLHttpRequest> {
+//   const req = new XMLHttpRequest()
+//   req.open("POST", url)
+//   req.setRequestHeader("Authorization", "Bearer " + token)
+//   return new Promise((res, rej) => {
+//     req.onreadystatechange = function() {
+//       if (this.readyState === 4) {
+//         if (this.status == 0) {
+//           return; // happens simultaneously with onerror
+//         }
+//         res(this)
+//       }
+//     }
+//     req.onerror = rej
+//     req.send(batch.buffer)
+//   })
+// }
+
+
 export default class QueueSender {
   private attemptsCount = 0
   private busy = false
@@ -24,7 +45,6 @@ export default class QueueSender {
     if (this.busy || !this.token) {
       this.queue.push(batch)
     } else {
-      this.busy = true
       this.sendBatch(batch)
     }
   }
@@ -40,6 +60,8 @@ export default class QueueSender {
 
   // would be nice to use Beacon API, but it is not available in WebWorker
   private sendBatch(batch: Uint8Array):void {
+    this.busy = true
+
     fetch(this.ingestURL, {
       body: batch,
       method: 'POST',
@@ -47,7 +69,7 @@ export default class QueueSender {
         "Authorization": "Bearer " + this.token,
         //"Content-Type": "",
       },
-      keepalive: true,
+      keepalive: batch.length < KEEPALIVE_SIZE_LIMIT,
     })
     .then(r => {
       if (r.status === 401) { // TODO: continuous session ?
@@ -69,8 +91,9 @@ export default class QueueSender {
       }
     })
     .catch(e => {
+      console.warn("OpenReplay:", e)
       this.retry(batch)
-    })     // Does it handle offline exceptions (?)
+    })
 
   }