I was using 0.9.11 and with scanners it was working fine. but by just changing the version to 0.9.12 , stared getting the exception for "org.reflections.RflectionException: Scanner SubTypeScanner was not configured".

Reflections reflections = new Reflections(new ConfigurationBuilder()
     .setUrls(ClasspathHelper.forPackage("my.project.prefix"))
     .setScanners(new SubTypesScanner(), new TypeAnnotationsScanner()),
     .filterInputsBy(new FilterBuilder().includePackage("my.project.prefix"))
     );

reflections.getSubTypesOf(someClass);  // here it throws exception 

I noticed that, when I add a class which extend some class to the package, this error goes.
Same with FieldAnnotationScanner , or any other scanner.

To try , please provide an invalid package for reflection creation, you will see all these errors coming even after configuring all scanners with reflection