--- a/src/hotspot/share/classfile/moduleEntry.cpp	Mon Jun 04 11:19:54 2018 +0200
+++ b/src/hotspot/share/classfile/moduleEntry.cpp	Mon Jun 04 11:07:04 2018 -0400
@@ -25,7 +25,7 @@
 #include "precompiled.hpp"
 #include "jni.h"
 #include "classfile/classLoaderData.inline.hpp"
-#include "classfile/javaClasses.hpp"
+#include "classfile/javaClasses.inline.hpp"
 #include "classfile/moduleEntry.hpp"
 #include "logging/log.hpp"
 #include "memory/resourceArea.hpp"
@@ -236,10 +236,17 @@
   // The java.lang.Module for this loader's
   // corresponding unnamed module can be found in the java.lang.ClassLoader object.
   oop module = java_lang_ClassLoader::unnamedModule(cld->class_loader());
+
+  // Ensure that the unnamed module was correctly set when the class loader was constructed.
+  // Guarantee will cause a recognizable crash if the user code has circumvented calling the ClassLoader constructor.
+  ResourceMark rm;
+  guarantee(java_lang_Module::is_instance(module),
+            "The unnamed module for ClassLoader %s, is null or not an instance of java.lang.Module. The class loader has not been initialized correctly.",
+            cld->loader_name());
+
   ModuleEntry* unnamed_module = new_unnamed_module_entry(Handle(Thread::current(), module), cld);
 
-  // Store pointer to the ModuleEntry in the unnamed module's java.lang.Module
-  // object.
+  // Store pointer to the ModuleEntry in the unnamed module's java.lang.Module object.
   java_lang_Module::set_module_entry(module, unnamed_module);
 
   return unnamed_module;

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/runtime/modules/ClassLoaderNoUnnamedModule.java	Mon Jun 04 11:07:04 2018 -0400
@@ -0,0 +1,55 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import jdk.internal.misc.Unsafe;
+
+public class ClassLoaderNoUnnamedModule {
+    static final Unsafe UNSAFE = Unsafe.getUnsafe();
+
+    class TestClass extends ClassLoader {
+        public boolean calledConstructor = false;
+
+        public TestClass() {
+            calledConstructor = true;
+        }
+    }
+
+    static void testConstructorCall() throws Exception {
+        // Use Unsafe allocateInstance to construct an instance of TestClass
+        // which does not invoke its super's, java.lang.ClassLoader, constructor.
+        // An unnamed module for this ClassLoader is not created.
+        TestClass tc = (TestClass)UNSAFE.allocateInstance(TestClass.class);
+        System.out.println("tc = " + tc + "tc's ctor called = " + tc.calledConstructor);
+        Module unnamed_module = tc.getUnnamedModule();
+        if (unnamed_module == null) {
+            System.out.println("The unnamed module for this class loader is null");
+        }
+
+        tc.loadClass(String.class.getName());
+        Class.forName(String.class.getName(), false, tc);
+    }
+
+    public static void main(String args[]) throws Exception {
+        testConstructorCall();
+    }
+}

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/runtime/modules/ClassLoaderNoUnnamedModuleTest.java	Mon Jun 04 11:07:04 2018 -0400
@@ -0,0 +1,49 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 8202758
+ * @summary Ensure that if the JVM encounters a ClassLoader whose unnamedModule field is not set an InternalError results.
+ * @modules java.base/jdk.internal.misc
+ * @library /test/lib
+ * @compile ClassLoaderNoUnnamedModule.java
+ * @run main/othervm ClassLoaderNoUnnamedModuleTest
+ */
+
+import jdk.test.lib.process.ProcessTools;
+import jdk.test.lib.process.OutputAnalyzer;
+
+public class ClassLoaderNoUnnamedModuleTest {
+    public static void main(String args[]) throws Throwable {
+        ProcessBuilder pb = ProcessTools.createJavaProcessBuilder(
+                               "--add-modules=java.base",
+                               "--add-exports=java.base/jdk.internal.misc=ALL-UNNAMED",
+                               "-XX:-CreateCoredumpOnCrash",
+                               "ClassLoaderNoUnnamedModule");
+        OutputAnalyzer oa = new OutputAnalyzer(pb.start());
+        oa.shouldContain("Internal Error");
+        oa.shouldContain("unnamed module");
+        oa.shouldContain("null or not an instance of java.lang.Module");
+    }
+}