A thing I do not understand

[Friedrich <frido q-software-solutions de>]

If I write  class which derives frm another and add this to  the
class_init method

static void qtestderived_class_init(gpointer g_class,
                             gpointer g_class_data){
  
  QTestDerivedClass *selfClass = QTESTDERIVED_CLASS(g_class);
  superClass = g_type_class_peek_parent(selfClass);


  g_type_class_add_private(selfClass, sizeof(struct QTestDerivedData));

  /* assign virtual methods  */
  selfClass->get_someIntVal = &qtestderived_getIntValImpl;
  selfClass->set_someIntVal = &qtestderived_setIntValImpl;
 }
  
I can later access superClass to call the parents method e.g like
this:
static int qtestderived_getIntValImpl(QTestDerived *self) {
  int result = superClass->get_someIntVal(QTEST(self));
  return  result;


}

However if I put all the handling for getting a superClass pointer in
this method like this:
static int qtestderived_getIntValImpl(QTestDerived *self) {
  QTestDerivedClass *selfClass = QTESTDERIVED_CLASS(self);
  QTestClass *sClass = g_type_class_peek_parent(selfClass);
  int result = sClass->get_someIntVal(QTEST(self));
  return  result;


}

Then the following happens, the stuff runs fine up to theline
QTESTDERVIVED_CLASS, but while calling g_type_clas...
I got back a NULL pointer. I simple do not understand why that
happens. The example in the docs looks like this:

The function g_type_class_peek_parent is used to access the original parent class structure. Its input is a pointer to the class of the derived object and it returns a pointer to the original parent class structure. The code below shows how you could use it:

static void
b_method_to_call (B *obj, int a)
{
  BClass *klass;
  AClass *parent_class;
  klass = B_GET_CLASS (obj);
  parent_class = g_type_class_peek_parent (klass);

  /* do stuff before chain up */
  parent_class->method_to_call (obj, a);
  /* do stuff after chain up */
}

So it looks very similiar... 

I've no idea where my thoughts go wrong.

Could anyone tell me about this?

Thanks in advance
Friedrich