What is the reason why classes with a recursive definition cannot be generated?

For example, the call to 'toClass()' in the following test case produces the exception stacktrace below :

import javassist.CannotCompileException;
import javassist.ClassPool;
import javassist.CtField;
import javassist.CtNewClass;
import javassist.NotFoundException;
import junit.framework.TestCase;

public class TestRecursiveClass extends TestCase {
 public void test() throws NotFoundException, CannotCompileException {
 ClassPool pool=ClassPool.getDefault();

 CtNewClass recursiveClass = new CtNewClass("RecursiveClass", pool , false, pool.get("java.lang.Object"));
 CtField f=new CtField(recursiveClass, "instance", recursiveClass);
 recursiveClass.addField(f, "null");

 recursiveClass.toClass();
 }
}

javassist.CannotCompileException: by javassist.CannotCompileException: cannot find RecursiveClass
at javassist.ClassPool.toClass(ClassPool.java:816)
at javassist.ClassPool.toClass(ClassPool.java:765)
at javassist.CtClass.toClass(CtClass.java:985)
at TestRecursiveClass.test(TestRecursiveClass.java:16)