src/java.base/share/classes/jdk/internal/loader/ClassLoaders.java

@@ -131,11 +131,12 @@
      * from the application class loader.
      */
     private static class ExtClassLoader extends BuiltinClassLoader {
         static {
             if (!ClassLoader.registerAsParallelCapable())
-                throw new InternalError();
+                throw new InternalError("Classloader is not registered" +
+                    " as parallel capable.");
         }
 
         ExtClassLoader(BootClassLoader parent) {
             super(parent, null);
         }

@@ -156,11 +157,12 @@
      * customizations to be compatible with long standing behavior.
      */
     private static class AppClassLoader extends BuiltinClassLoader {
         static {
             if (!ClassLoader.registerAsParallelCapable())
-                throw new InternalError();
+                throw new InternalError("Classloader is not registered" +
+                    " as parallel capable.");
         }
 
         final URLClassPath ucp;
 
         AppClassLoader(ExtClassLoader parent, URLClassPath ucp) {