diff --git a/script.lua b/script.lua
index 60d10b0..67f9151 100644
--- a/script.lua
+++ b/script.lua
@@ -619,7 +619,11 @@ function internal.write_classified_node(vm_data, va, used_biomes, classified_nod
     local default_biome = internal.default_biome()
 
     local biome_to_id = {}
-    biome_to_id[default_biome.name] = default_biome[biome_key] or ""
+    if default_biome[biome_key] == nil then
+        biome_to_id[default_biome.name] = ""
+    else
+        biome_to_id[default_biome.name] = minetest.get_content_id(default_biome[biome_key])
+    end
 
     for _, i in ipairs(classified_nodes) do
         local pos = small_va:position(i)