Can't start feature branch off of another feature branch

I'm trying to do sub-feature branches on a feature branch using embedded git-flow and keep getting an error that the base "needs to be a branch. It does not exist and is required."

Prerequisities:

git-flow configured through Sourcetree.

Steps to reproduce:

1. make sure you are on a feature branch that has at least on commit (FC1)
2. tap "git-flow" button in the toolbar (it should offer to complete a feature)
3. Choose "Start Feature" from "other actions" drop down.
4. In opened dialog box select "Working copy revision" (which is FC1)
5. Type any feature name
6. Click "Ok"

The following error text is displayed in disclosed log text area:

git feature start presentation {FC1 sha} 
Fatal: Base '{FC1 sha}' needs to be a branch. It does not exist and is required.
Completed with errors, see above

Error can also be reproduced by selecting "Specified commit" and picking FC1 commit from the commit dialog box.

macOS: 10.15.1
system git: 2.21.0
embeded git: 2.20.1