Sphinx Doctest Not Finding Tests

I'm trying to figure out how to get doctest working with Sphinx, but it doesn't seem to be finding my tests. I have the following simple example.

def my_func():
    '''
    Dummy test function. Returns the number 5.
    .. doctest::

    >>> my_func()
    5
    '''
    return 5

When I run make doctest, the output tells me that there were zero tests. I'm pretty sure I have things configured correctly because if I run make html and then go to my index.html file, I see the function my_func() included in the documentation.

Am I overlooking something simple here? Thanks for your help.