It seems the project uses a version of aleph that is not compatible with Java 9 . It has been fixed with 0.4.5-alpha1 (https://github.com/ztellman/aleph/commit/b4a49a878d73bb7db4178f7e62ddc9feb912f664). This issue was filed with data from Clojars.org . Feel free to close this if it's irrelevant.