How to use mockery on private classes?

I want to write an unit test for a method which is using private method getRandNumber(). In my test getRandNumber() should always return 1.

How can I mock this method?

I've tried

$myModel = $this->createPartialMock(MyModel::class, ['getRandNumber']);

$reflector = new \ReflectionClass(MyModel::class);
$m = $reflector->getMethod('getRandNumber');
$m->setAccessible(true);

$myModel->method('getRandNumber')->willReturn(1);

getting error

  [PHPUnit\Framework\Warning] Trying to configure method "getRandNumber" which cannot be configured because it does not exist, has not been specified, is final, or is static