"edit-tag" endpoint does not seem to work

[nearestneighbour]

I'm trying to mark an item as read using the edit-tag endpoint. It seems like I'm doing everything right, the API calls where I obtain a list of unread items and their IDs and contents all work as expected. But when I call edit-tag with the parameters:

output=json
i=(item id)
a=user/-/state/com.google/read

I get a response with status code 200 and the text "OK", which sounds like it worked. But then I open the theoldreader.com GUI and the item is still unread, so the API call had no effect.

The same goes for starring and liking an item.