diff --git a/gridlock/grid.py b/gridlock/grid.py
index d4ae0b2..9c52f9a 100644
--- a/gridlock/grid.py
+++ b/gridlock/grid.py
@@ -587,7 +587,7 @@ class Grid(object):
                          [+dimensions[0], +dimensions[1]],
                          [+dimensions[0], -dimensions[1]],
                          [-dimensions[0], -dimensions[1]]], dtype=float)
-        thickness = dimensions[3]
+        thickness = dimensions[2]
         self.draw_polygon(Direction.z, center, p, thickness, eps)
 
     def draw_cylinder(self,