Through the api, &fields=excerpt doesn't work?

So I’m prepping ghost to serve as a blog/news post administration capacity to a pre-existing flask+jinja based site. Mostly, this is going well.

I did experience something unexpected, however.

    def get_features( self, id = None, limit = 3 ):
        url = f"/ghost/api/v3/content/posts/?key={ self.api_key }&filter=tag:hash-feature&fields=custom_excerpt,title,url,feature_image&limit={ limit }"
        return requests.get( + url ).json()

This works fine. But originally I tried to include &fields=excerpt,custom_excerpt,url,feature_image … and that also worked… except that excerpt was not included in the results. When I do not include any fields at all, I see that excerpt is sent alongside the custom_excerpt, but it’s ignored when requested specifically. In fact, when I ask ONLY for excerpt, ghost results no results at all… not even the 3 ready and waiting items.

Am I misunderstanding the excerpt field somehow? I’d hoped to have the ability to fail over to the excerpt in the case that one of our staff forgets to create a custom one and publishes theme-breaking lack of text