Simple prolog program. Getting error: >/2: Arguments are not sufficiently instantiated

I made a Prolog predicate posAt(List1,P,List2) that tests whether the element at position P of List1 and List2 are equal:

posAt([X|Z], 1, [Y|W]) :-
   X = Y.
posAt([Z|X], K, [W|Y]) :-
   K > 1,
   Kr is K - 1,
   posAt(X, Kr, Y).

When testing:

?- posAt([1,2,3], X, [a,2,b]).

I expected an output of X = 2 but instead I got the following error:

ERROR: >/2: Arguments are not sufficiently instantiated

Why am I getting this error?