diff --git a/src/lib/lists.pl b/src/lib/lists.pl
index 22ee8d4b..964280d7 100644
--- a/src/lib/lists.pl
+++ b/src/lib/lists.pl
@@ -54,7 +54,7 @@ length(Xs0, N) :-
    '$skip_max_list'(M, N, Xs0,Xs),
    !,
    (  Xs == [] -> N = M
-   ;  nonvar(Xs) -> var(N), throw(error(resource_error(finite_memory),length/2))
+   ;  nonvar(Xs) -> var(N), Xs = [_|_], throw(error(resource_error(finite_memory),length/2))
    ;  nonvar(N) -> R is N-M, length_rundown(Xs, R)
    ;  N == Xs -> throw(error(resource_error(finite_memory),length/2))
    ;  length_addendum(Xs, N, M)