Can't set external wiki url

I was trying to set an external wiki url for my repository via api. Using Swagger the following curl command should edit my repository:

curl -X PATCH "http://localhost:3000/api/v1/repos/gitea/test" -H "accept: application/json" -H "authorization: Basic replace=" -H "Content-Type: application/json" -d "{ \"external_wiki\": { \"external_wiki_url\": \"\" }}"

But nothing changes. When navigating via browser to http://localhost:3000/gitea/test/settings, I’m able to change the external wiki uri manually.

Using gitea api: 1.1.1, gitea version: 1.13.0

Can you give me a hint, how I can change my external wiki url via api?