diff --git a/src/brep/step_import.cpp b/src/brep/step_import.cpp
index 5148c70..74a8bb5 100644
--- a/src/brep/step_import.cpp
+++ b/src/brep/step_import.cpp
@@ -3,6 +3,7 @@
 #include "vde/curves/nurbs_surface.h"
 #include <fstream>
 #include <sstream>
+#include <optional>
 #include <unordered_map>
 #include <unordered_set>
 #include <variant>
@@ -1115,7 +1116,7 @@ private:
         int loop_ref = -1;
         for (const auto& arg : ent.args) {
             int r = to_ref(arg);
-            if (r > 0 && entities_.count(r) && entities_[r].type == "EDGE_LOOP") {
+            if (r > 0 && entities_.count(r) && entities_.at(r).type == "EDGE_LOOP") {
                 loop_ref = r;
                 break;
             }
@@ -1162,7 +1163,7 @@ private:
         bool reversed = false;
         for (const auto& arg : ent.args) {
             int r = to_ref(arg);
-            if (r > 0 && entities_.count(r) && (entities_[r].type == "EDGE_CURVE" || entities_[r].type == "EDGE_CURVE_IMPLICIT")) {
+            if (r > 0 && entities_.count(r) && (entities_.at(r).type == "EDGE_CURVE" || entities_.at(r).type == "EDGE_CURVE_IMPLICIT")) {
                 edge_ref = r;
             }
         }
@@ -1332,7 +1333,7 @@ private:
             std::vector<int> filtered;
             for (int r : solid_refs) {
                 if (entities_.count(r)) {
-                    const auto& e = entities_[r];
+                    const auto& e = entities_.at(r);
                     if (e.type == "MANIFOLD_SOLID_BREP" || e.type == "SHELL_BASED_SURFACE_MODEL")
                         filtered.push_back(r);
                 }
@@ -1352,10 +1353,10 @@ private:
             if (arg.type == StepValue::Type::Ref) {
                 int r = arg.ival;
                 if (entities_.count(r)) {
-                    if (entities_[r].type == target_type)
+                    if (entities_.at(r).type == target_type)
                         out.push_back(r);
                     else
-                        find_nested_refs(entities_[r], target_type, out);
+                        find_nested_refs(entities_.at(r), target_type, out);
                 }
             } else if (arg.type == StepValue::Type::List) {
                 for (const auto& v : arg.list) {