What is this mistake "Illegal string offset 'id'"?

I want to get the value of json for example

function notification(Request $request){
    $registers =$request->get('json');
    return $newRegisters[0]["id"];
}

But I get this error:""Illegal string offset 'id'"

This method is called for post:

<input type="hidden" name="json[]" value="">

and the route is:

$app->post('notification','AtlanticControllerChangePrice@notification');

via Chebli Mohamed