Illegal argument in isolate message: object is unsendable Flutter / Dart Origin: On your device OSI layer 7 · Application Last updated 4 Nov 2026

Dart: Invalid argument(s): Illegal argument in isolate message: object is unsendable - Library:'dart:async' Class: _Future@4048458

You tried to send an object (closure, Future, socket, platform object) to another isolate that cannot be copied.

Exact message you may see Illegal argument in isolate message: object is unsendableInvalid argument(s): Illegal argument in isolate messageobject is unsendable - Library:'dart:async'Illegal argument in isolate message: (object extends NativeWrapper - Library:'dart:io')
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Seen on: Flutter

Meaning

compute(), Isolate.run and SendPort.send copy data. Only primitives, lists, maps, and simple data classes are allowed; closures capturing BuildContext/this, Futures, streams and native handles are not. Pass plain data and recreate objects inside the isolate.

Common causes

  • Closure capturing a non-sendable object (BuildContext, State)
  • Passing a Future/Stream/Socket
  • Sending a class with a native handle
  • compute() with a callback that is not top-level
  • Sending an object containing a ReceivePort

⚡ Quick fix

  1. Use a top-level or static function with plain arguments
  2. Extract the needed fields into a simple map or data class
  3. Don't capture this/context in the closure

Detailed fix by platform

Dart

  1. final result = await compute(parseJson, body); where List<Item> parseJson(String s) => … is top-level

How to diagnose

  1. Argument — Contains closure/context/Future?
  2. Function — Top-level?
  3. Data — Plain types?

🔧 Still not fixed?

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Where this error sits

Origin: On your device. Errors that are often confused with or caused by this one (click to open):

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