testing with phpspec - mocking method from the current class

I'm learning phpspec and can't figure out why a test is not passing.

Here is my function:

public function isTaskForChange($task)
{
    $supportedTasks = array_keys($this->availableTasks()); 
    $isTaskForChange = in_array($task, $supportedTasks);

    return $isTaskForChange;
}

And here is the test in phpspec:

public function it_validates_if_task_should_be_changed()
{
    $this->isTaskForChange('write')->shouldReturn(true);
}

However, when I run this code I get back:

warning: array_keys() expects parameter 1 to be array, null given

My question is: How to mock $this->availableTasks() to return values?