Asana api lists unassigned tasks in the workspace

I would like to list all unselected tasks in a given workspace using the asana API. Tried to use "assignee = null" as follows:

curl -u <api-key>: "https://app.asana.com/api/1.0/tasks?workspace=<id>&assignee=null"

but it returns an error below:

{
  "errors": [
    {
      "message": "assignee: Not an email, ID, or \"me\": null"
    }
  ]
}

In fact, any attempt to access tasks in the workspace without an explicit assignee (I or the identifier of the assignee) fails, examples

curl -u <api-id>: "https://app.asana.com/api/1.0/tasks?workspace=<wid>&due_on=null&opt_pretty" 
curl -u <api-id>: "https://app.asana.com/api/1.0/tasks?workspace=<wid>&opt_fields=name,notes,assignee"

all this error message

{
  "errors": [
    {
      "message": "assignee: Missing input"
    }
  ]
}

Unassigned tasks have a zero assignee when I list them, what can I do for a project, but not for a workspace:

curl -u <api-key>: "https://app.asana.com/api/1.0/projects/<pid>/tasks?opt_fields=name,assignee,notes"

{
  "id": 123456789,
  "name": "watch asana vision talk",
  "assignee": null,
  "notes": "http://blog.asana.com/2011/02/asana-demo-vision-talk/"
}

But I can’t list them from the project either since the recipient parameters are ignored (see also Asana API for projects assigned to me )

Am I missing something? Help rate! Thank.