diff --git a/init.lua b/init.lua
index 2294b09..e60370b 100644
--- a/init.lua
+++ b/init.lua
@@ -625,14 +625,17 @@ function internal.generate_caves(data, minp, maxp)
         internal.humidity_noise_params(), bminp, bmaxp
     )
 
+    local air = minetest.get_content_id("air")
+
     -- Place blocks where necessary
     internal.iter_3d_area(bminp, bmaxp, function (i, pos)
+        
         local function place(name)
-            if type(name) == "string" then
-                vmanip:set_index(
-                    vmanip:pos_to_index(pos),
-                    minetest.get_content_id(name)
-                )
+            local vi = vmanip:pos_to_index(pos)
+
+            if vmanip:get_index(vi) == air then
+            elseif type(name) == "string" then
+                vmanip:set_index(vi, minetest.get_content_id(name))
             elseif type(name) == "nil" then
             else
                 error("Inserted invalid type " .. type(name) .. " into voxelmanip array")