Trouble with py-fill-paragraph

Hello,

I'm trying to refill the following docstring:

class BlockCache(object):

    def remove(self, inode, start_no, end_no=None):
        """Remove blocks for `inode`

        If `end_no` is not specified, remove just the `start_no`
        block.
        Otherwise removes all blocks from `start_no` to, but not
        including, `end_no`.

        Note: if `get` and `remove` are called concurrently, then it is
        possible that a block that has been requested with `get` and
        passed to `remove` for deletion will not be deleted.
        """

        log.debug('remove(inode=%d, start=%d, end=%s): start', inode, start_no, end_no)

        if end_no is None:
            end_no = start_no + 1

When I place the cursor on e.g. the first line ("If `end_no`...) and execute M-x py-fill-paragraph, the buffer is scrolled such that this becomes the first visible line, and the indentation is removed. No filling occurs at all.

Am I doing something wrong? py-docstring-style is set to pep-256-nn.