Runtime error in ideal Coefficient ring of LHS must be a field

Question

I am trying the following code in magma

   z:=Integers();
   P<x>:=PolynomialRing(z);
   ideal<P|x>;

I tried also :

 z:=GR(2,2,1);
 P<x>:=PolynomialRing(z);
 ideal<P|x>;

both are shown the same erreur "Runtime error in ideal< ... >: Coefficient ring of LHS must be a field" Im trying to construct an ideal of Z[x]