Why can't I add a specific art item to my private collection?

Why can't I add a specific art item to my private collection?

bobjh's picture

I'm trying to add the following page:

low poly Lantern Game asset
https://opengameart.org/content/low-poly-lantern-game-asset

to an unlisted collection, but it generates the error message:

"Collected Art: found no valid post with that title."

I tried changing it to all lower case, but received the same error. Is there a work-around? Thanks.