We've been getting internal errors running jdk9 javadoc for a while now.

(cvs -d ':pserver:anonymous:@gee.cs.oswego.edu/home/jsr166/jsr166' checkout jsr166 && cd jsr166 && make -v docs)

But here's a simpler repro recipe:

 ~/ws/jdk9/jdk $ (rm -rf /tmp/t9; mkdir -p /tmp/t9; ../build/linux-x86_64-normal-server-release/images/jdk/bin/javadoc -d /tmp/t9 -sourcepath src/java.base/share/classes $(find -name ArrayDeque.java))
Loading source file ./src/java.base/share/classes/java/util/ArrayDeque.java...
java.lang.AssertionError
	at com.sun.tools.javac.util.Assert.error(jdk.compiler@9-internal/Assert.java:155)
	at com.sun.tools.javac.util.Assert.checkNull(jdk.compiler@9-internal/Assert.java:54)
	at com.sun.tools.javac.code.Symtab.enterModule(jdk.compiler@9-internal/Symtab.java:755)
	at com.sun.tools.javac.comp.Modules.enterModule(jdk.compiler@9-internal/Modules.java:300)
	at com.sun.tools.javac.comp.Modules.enterModules(jdk.compiler@9-internal/Modules.java:278)
	at com.sun.tools.javac.comp.Modules.enter(jdk.compiler@9-internal/Modules.java:246)
	at com.sun.tools.javac.comp.Modules.enter(jdk.compiler@9-internal/Modules.java:229)
	at com.sun.tools.javac.main.JavaCompiler.readSourceFile(jdk.compiler@9-internal/JavaCompiler.java:817)
	at com.sun.tools.javac.main.JavaCompiler.readSourceFile(jdk.compiler@9-internal/JavaCompiler.java:779)
	at com.sun.tools.javac.main.JavaCompiler.access$100(jdk.compiler@9-internal/JavaCompiler.java:98)
	at com.sun.tools.javac.main.JavaCompiler$1.complete(jdk.compiler@9-internal/JavaCompiler.java:340)
	at com.sun.tools.javac.code.ClassFinder.fillIn(jdk.compiler@9-internal/ClassFinder.java:361)
	at com.sun.tools.javac.code.ModuleFinder.lambda$findSingleModule$0(jdk.compiler@9-internal/ModuleFinder.java:206)
	at com.sun.tools.javac.code.Symbol.complete(jdk.compiler@9-internal/Symbol.java:610)
	at com.sun.tools.javac.comp.Modules.setCompilationUnitModules(jdk.compiler@9-internal/Modules.java:404)
	at com.sun.tools.javac.comp.Modules.enter(jdk.compiler@9-internal/Modules.java:248)
	at com.sun.tools.javac.comp.Modules.initModules(jdk.compiler@9-internal/Modules.java:214)
	at jdk.javadoc.internal.tool.ElementsTable.scanSpecifiedItems(jdk.javadoc@9-internal/ElementsTable.java:406)
	at jdk.javadoc.internal.tool.JavadocTool.getEnvironment(jdk.javadoc@9-internal/JavadocTool.java:181)
	at jdk.javadoc.internal.tool.Start.parseAndExecute(jdk.javadoc@9-internal/Start.java:406)
	at jdk.javadoc.internal.tool.Start.begin(jdk.javadoc@9-internal/Start.java:295)
	at jdk.javadoc.internal.tool.Start.begin(jdk.javadoc@9-internal/Start.java:239)
	at jdk.javadoc.internal.tool.Main.execute(jdk.javadoc@9-internal/Main.java:63)
	at jdk.javadoc.internal.tool.Main.main(jdk.javadoc@9-internal/Main.java:52)
javadoc: error - fatal error