The section "4.1 ClassFile" of JVM spec says about super_class item:

"If the value of super_class is zero, then this class file must represent
the class java.lang.Object."

While loading class that is not java.lang.Object and that has super_class
item equal to zero the exception UnknownError is thrown.

To reproduce run JCK test
   tests/vm/classfmt/clf/clfsup004/clfsup00401/clfsup00401.html