Terminate Dos Subst with Delphi

In the Delphi 2.0 help file this description is given on how to remove
a DOS substituted drive from Win32:

> mov ax, 71aah            ; SUBST
> mov bh, 1                ; Terminate Subst
> mov bl, DriveNum         ; see below
> int 21h
> jc error
> Terminate the association between a path and a drive letter.
> Clears the carry flag if successful. Otherwise, the function sets the carry flag and returns an error value in the AX register.
> DriveNum
> Drive to terminate SUBST. This parameter can 1 for A, 2 for B, and so on. Note that DriveNum cannot be 0 to indicate the default drive.

When I include this code in asm .... end my program crashes saying that
an illegal instruction has taken place. Why?

Jimmy Thomsen.