Z Transform problem ?

[Natoe]

Hi !

Today in my school i made many exercice with the Z transform like the basic :1 -> z/z-1

So in the hp-prime i use this command : ztrans(1,n,z) and its show me a beautiful : z/z-1

Now i want to do the "inverse z transform"

So i use the command : invztrans(z/z-1,z,n) and its show me 1

every things is fine , but ...

when i used invztrans(2/z-3,z,n) , its show me (-2*Dirac(n)+2*3^n)/3 ... Wtf ?

its not the right answer , what is the problème ? :/

Thanks