Building with JDK 12 reports the following error:

The code being documented uses modules but the packages defined in https://docs.oracle.com/javase/8/docs/api/ are in the unnamed module

This fails the build, and seem related to https://bugs.openjdk.java.net/browse/JDK-8212233

Apparently the same problem can also be reproduced by some JDK 11 versions too.

Solution seems simple: specify --source 8 on the javadoc tool.