diff --git a/init.lua b/init.lua
index 18bf171..9438aa0 100644
--- a/init.lua
+++ b/init.lua
@@ -414,6 +414,9 @@ function internal.clean_shape_def(def)
         y_max = def.y_max or internal.world_maxp.y,
     }
 
+    d.y_min = internal.reduced_shape_pos({ x = 0, y = d.y_min, z = 0 }).y + 1
+    d.y_max = internal.reduced_shape_pos({ x = 0, y = d.y_max, z = 0 }).y - 1
+
     if type(def.noise_params) == "table" then
         d.noise_params = def.noise_params
     end