Why does my jar file not not contain any class files?

I'm trying to add a task (gen or gen2) to my build.gradle that does exactly the same as the Jar-task:

version = "0.0.1"
apply plugin: 'java'

task('gen', type: Jar) {
}

task gen2(type: Jar)

Running

gradle jar

generates a JAR-file that contains .class-files, while running

gradle gen

or

gradle gen2

generate a JAR-file that does NOT contain any .class-files.

Whats wrong with my class definition?