The NullPointerException duplicates with jdk9/dev but it's possible to get more information with jigsaw/jake builds due improved exceptions and error reporting.

$ jlink -J-Djlink.debug=true --output myimage --include-locales=en-US --compress=2 --strip-debug --vm=minimal --modulepath jmods --addmods java.base

Error: java.lang.NullPointerException
java.io.IOException: java.lang.NullPointerException
at jdk.tools.jlink.internal.ImageFileCreator.generateJImage(jdk.jlink@9-internal/ImageFileCreator.java:190)
at jdk.tools.jlink.internal.ImageFileCreator.writeImage(jdk.jlink@9-internal/ImageFileCreator.java:168)
at jdk.tools.jlink.internal.ImageFileCreator.create(jdk.jlink@9-internal/ImageFileCreator.java:103)
at jdk.tools.jlink.internal.JlinkTask$ImageHelper.retrieve(jdk.jlink@9-internal/JlinkTask.java:506)
at jdk.tools.jlink.internal.ImagePluginStack.operate(jdk.jlink@9-internal/ImagePluginStack.java:207)
at jdk.tools.jlink.internal.JlinkTask.createImage(jdk.jlink@9-internal/JlinkTask.java:335)
at jdk.tools.jlink.internal.JlinkTask.run(jdk.jlink@9-internal/JlinkTask.java:209)
at jdk.tools.jlink.internal.Main.main(jdk.jlink@9-internal/Main.java:33)
Caused by: java.lang.NullPointerException
at jdk.tools.jlink.internal.plugins.IncludeLocalesPlugin.previsit(jdk.jlink@9-internal/IncludeLocalesPlugin.java:180)
at jdk.tools.jlink.internal.ImagePluginStack.visitResources(jdk.jlink@9-internal/ImagePluginStack.java:244)
at jdk.tools.jlink.internal.ImageFileCreator.generateJImage(jdk.jlink@9-internal/ImageFileCreator.java:186)
... 7 more