Let's consider following directory structure:

mod\module-info.java:
module mod {
    exports pkg;
}

mod\pkg\module-info.java:
package pkg;

class C {
}

When it's compiled by [1] in a following manner:

javac mod\module-info.java mod\pkg\module-info.java

the output below is produced:

An exception has occurred in the compiler (9-ea). Please file a bug against the Java compiler via th
e Java bug reporting page (http://bugreport.java.com) after checking the Bug Database (http://bugs.j
ava.com) for duplicates. Include your program and the following diagnostic in your report. Thank you
.
java.lang.NullPointerException
        at com.sun.tools.javac.comp.TypeEnter$ImportsPhase.checkClassPackageClash(jdk.compiler@9-ea/
TypeEnter.java:362)
        at com.sun.tools.javac.comp.TypeEnter$ImportsPhase.resolveImports(jdk.compiler@9-ea/TypeEnte
r.java:342)
        at com.sun.tools.javac.comp.TypeEnter$ImportsPhase.runPhase(jdk.compiler@9-ea/TypeEnter.java
:304)
        at com.sun.tools.javac.comp.TypeEnter$Phase.doCompleteEnvs(jdk.compiler@9-ea/TypeEnter.java:
266)
        at com.sun.tools.javac.comp.TypeEnter$Phase.completeEnvs(jdk.compiler@9-ea/TypeEnter.java:24
4)
        at com.sun.tools.javac.comp.TypeEnter.complete(jdk.compiler@9-ea/TypeEnter.java:198)
        at com.sun.tools.javac.code.Symbol.complete(jdk.compiler@9-ea/Symbol.java:601)
        at com.sun.tools.javac.code.Symbol$ClassSymbol.complete(jdk.compiler@9-ea/Symbol.java:1220)
        at com.sun.tools.javac.comp.Enter.complete(jdk.compiler@9-ea/Enter.java:547)
        at com.sun.tools.javac.comp.Enter.main(jdk.compiler@9-ea/Enter.java:524)
        at com.sun.tools.javac.main.JavaCompiler.enterTrees(jdk.compiler@9-ea/JavaCompiler.java:1053
)
        at com.sun.tools.javac.main.JavaCompiler.compile(jdk.compiler@9-ea/JavaCompiler.java:917)
        at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-ea/Main.java:261)
        at com.sun.tools.javac.main.Main.compile(jdk.compiler@9-ea/Main.java:143)
        at com.sun.tools.javac.Main.compile(jdk.compiler@9-ea/Main.java:55)
        at com.sun.tools.javac.Main.main(jdk.compiler@9-ea/Main.java:41)

The minimized testcase is attached; in order to run it please:

1. Unzip the attached archive to some dir on Windows machined, say directory A;
2. Rename A\test\test_bat to A\test\test.bat;
3. Modify test.bat by changing JDK_HOME variable to point to your jigsaw JDK 9 installation directory;
4. Run test.bat.

[1] http://download.java.net/java/jigsaw/archive/106/binaries/jigsaw-jdk-9-ea+106_windows-x86_bin.zip