Illegal string offset in laravel [closed]

I have this error:

  

ErrorException in PuntuacionesCtrl.php line 70:   Illegal string offset 'activity_id'

and I do not know how to solve it, this is my code, the first line is 70 where I get the error:

    $actividad_id = $request->detalle['actividad_id'];
    $puntos = $request->detalle['puntos'];
    $cont= 0;
    while($cont < count($actividad_id)){
        $detalle = new Detalle();
        $detalle->puntuacion_id= $puntuacion->id;
        $detalle->actividad_id= $actividad_id[$cont];
        $detalle->puntos= $puntos[$cont];
        $detalle->save();

        $cont=$cont + 1;
    }

so I have it in the form:

<select multiple class="form-control" name="actividad_id" ng-model="puntuacion.detalle.actividad_id" ng-options="actividad.id as actividad.nombre for actividad in actividades | filter:{ circular_id: puntuacion.circular_id, tipo_a_id: 1} "></select>

I did:

 dump($request->detalle);

and I get [object Object] here the image: