diff --git a/library/include/dfhack/modules/Materials.h b/library/include/dfhack/modules/Materials.h
index 91e1f12c1..50142af4a 100644
--- a/library/include/dfhack/modules/Materials.h
+++ b/library/include/dfhack/modules/Materials.h
@@ -500,26 +500,27 @@ namespace DFHack
         int16_t clustersize;
         std::vector<std::string *> prefstring;
         std::vector<df_material *> materials;
-        int16_t material_subid_basic_mat;
-        int16_t material_subid_tree;
-        int16_t material_subid_drink;
-        int16_t material_subid_seed;
-        int16_t material_subid_thread;
-        int16_t material_subid_mill;
-        int16_t material_subid_extract_vial;
-        int16_t material_subid_extract_barrel;
-        int16_t material_subid_extract_still_vial;
-        int16_t material_subid_leaves;
-        int32_t material_id_basic_mat;
-        int32_t material_id_tree;
-        int32_t material_id_drink;
-        int32_t material_id_seed;
-        int32_t material_id_thread;
-        int32_t material_id_mill;
-        int32_t material_id_extract_vial;
-        int32_t material_id_extract_barrel;
-        int32_t material_id_extract_still_vial;
-        int32_t material_id_leaves;
+        // materials and material indexes - only valid when appropriate flags in the BitArray are set
+        int16_t material_basic_mat;
+        int16_t material_tree;
+        int16_t material_drink;
+        int16_t material_seed;
+        int16_t material_thread;
+        int16_t material_mill;
+        int16_t material_extract_vial;
+        int16_t material_extract_barrel;
+        int16_t material_extract_still_vial;
+        int16_t material_leaves;
+        int32_t material_idx_basic_mat;
+        int32_t material_idx_tree;
+        int32_t material_idx_drink;
+        int32_t material_idx_seed;
+        int32_t material_idx_thread;
+        int32_t material_idx_mill;
+        int32_t material_idx_extract_vial;
+        int32_t material_idx_extract_barrel;
+        int32_t material_idx_extract_still_vial;
+        int32_t material_idx_leaves;
         std::string material_str_basic_mat[3];
         std::string material_str_tree[3];
         std::string material_str_drink[3];