Cannot delete public contribution

An API call to:

DELETE /api/v1/toolchains/tutorial/bla/2/

Gets me a 403 error and a message saying the toolchain is being used. I'm sure it is not. I've reproduced this on the staging system and on my local machine.

Something is not being calculated right under the API.

How to:

1. Create a new toolchain and make it public
2. Try to delete it

The system says it can't, because it's being used (while it's not). It should allow me to delete this contribution. This should be valid for all unused contributions independent of their sharing level.