weird error when trying to build example-kotlin repo:

ClasspathRootOnlyWasInner: The following targets had dependees, but can only be used as roots in a build graph:
  * src/jvm/org/pantsbuild/example/json/JsonExample.kt	(kotlin_source)

https://github.com/pantsbuild/example-kotlin/runs/6438158675?check_suite_focus=true