diff --git a/day17.py b/day17.py
index 31937d7..c568db8 100644
--- a/day17.py
+++ b/day17.py
@@ -22,7 +22,7 @@ def simulate(active_cubes, fourD=False):
                         if not (x == cx and y == cy and z == cz):
                             active_neighbours[(cx, cy, cz)] += 1
                     else:
-                        for cw in [active_cube[3] - 1, active_cube[3], active_cube[3] + 1]:
+                        for cw in [w - 1, w, w + 1]:
                             if not (x == cx and y == cy and z == cz and w == cw):
                                 active_neighbours[(cx, cy, cz, cw)] += 1