Confusion with FORTRAN 2008 OO

The more I use F08 object orientation, the more confused I get... I was pretty sure I had tested this previously with Intel compiler and it worked (about 4 years ago) but now it refuses to!

Imagine a simple linked list implementation:

TYPE :: List_T
  CLASS(List_T), POINTER :: next => null()
  CLASS(List_T), POINTER :: prev => null()

  CONTAINS
    PROCEDURE, PUBLIC :: insert  => ListInsert
    PROCEDURE, PUBLIC :: first   => ListFirst
    PROCEDURE, PUBLIC :: last    => ListLast
    PROCEDURE, PUBLIC :: found   => ListFound
    PROCEDURE, PUBLIC :: append  => ListAppend
    PROCEDURE, PUBLIC :: print   => ListPrint
    FINAL :: ListFree
END TYPE List_T

SUBROUTINE ListAppend(this,new)
  CLASS(List_T), INTENT(INOUT), TARGET  :: this
  CLASS(List_T), INTENT(IN),    POINTER :: new
  ! do stuff here
END SUBROUTINE ListAppend

Then I create a type ExtList_T which extends List_T. I don't want to redefine the "append" function because it is exactly the same, I just want to inherit it. But then if I try something like

TYPE(ExtList_T), POINTER  :: instance1
TYPE(ExtList_T), POINTER  :: instance2

ALLOCATE(instance1)
ALLOCATE(instance2)
! put stuff inside each
CALL instance1%append(instance2)

Intel Compiler complains "error #6633: The type of the actual argument differs from the type of the dummy argument.   [INSTANCE2]"

gfortran compiles that fine. Am I doing something conceptually wrong? Isn't downcasting like that allowed in Fortran?