crash deallocating passed pointers

We want to create a structure holding strings that vary greatly in length that can be modified during program execution. So we are using deferred length pointers. However, we are running into a problem deallocating them later. Below is a greatly simplified program that demonstrates the issue. The deallocate in the main program works. The one in subroutine a crashes.

      
      character(:), pointer :: ptr1
      character(:), pointer :: ptr2
      integer ierr
      
      allocate(character(12) :: ptr1, stat = ierr)
      ptr1 = "ABCDEFGHIJKLM"
      ptr2 => ptr1
      deallocate(ptr2)
      allocate(character(12) :: ptr1, stat = ierr)
      ptr1 = "MLKJIHGFEDCBA"
      call a(ptr1)
      
      contains
      
      subroutine a(ptr)
      character(:), pointer :: ptr
      
      ptr2 => ptr
      deallocate(ptr2)
      
      end subroutine a
      end