My Yahoo doesn’t see feed

Here is the rss feed:
https://markhillman.com/feed/

I click on it.
I get subscribe to feed using with a drop down box.
I choose yahoo and click subscribe now.
I go to a page that says: Get https://markhillman.com/feed/ on My Yahoo! and see the list of recent articles.
Then I click Add to My Yahoo and get this:

We couldn’t find the RSS file you asked for https://markhillman.com/feed/
Suggestions:

* – If you got here by clicking on an “Add to My” button, please notify the referring site’s webmaster.
* – If you entered the URL yourself, please check that the URL is correct and points to a valid RSS file.

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I don’t know what the problem is.