diff --git a/code/compiler/13/parsed_type.cpp b/code/compiler/13/parsed_type.cpp
index d119509..2fa31ee 100644
--- a/code/compiler/13/parsed_type.cpp
+++ b/code/compiler/13/parsed_type.cpp
@@ -9,10 +9,10 @@ type_ptr parsed_type_app::to_type(
         const type_env& e) const {
     auto parent_type = e.lookup_type(name);
     if(parent_type == nullptr)
-        throw type_error(std::string("no such type or type constructor ") + name);
+        throw type_error("no such type or type constructor " + name);
     type_base* base_type;
     if(!(base_type = dynamic_cast<type_base*>(parent_type.get())))
-        throw type_error(std::string("invalid type ") + name);
+        throw type_error("invalid type " + name);
     if(base_type->arity != arguments.size()) {
         std::ostringstream error_stream;
         error_stream << "invalid application of type ";
@@ -34,7 +34,7 @@ type_ptr parsed_type_var::to_type(
         const std::set<std::string>& vars,
         const type_env& e) const {
     if(vars.find(var) == vars.end())
-        throw type_error(std::string("the type variable ") + var + std::string(" was not explicitly declared."));
+        throw type_error("the type variable " + var + " was not explicitly declared.");
     return type_ptr(new type_var(var));
 }
 
diff --git a/code/compiler/13/type_env.cpp b/code/compiler/13/type_env.cpp
index 0c0ddef..47a5212 100644
--- a/code/compiler/13/type_env.cpp
+++ b/code/compiler/13/type_env.cpp
@@ -1,6 +1,7 @@
 #include "type_env.hpp"
 #include "type.hpp"
 #include "error.hpp"
+#include <cassert>
 
 void type_env::find_free(const type_mgr& mgr, std::set<std::string>& into) const {
     if(parent != nullptr) parent->find_free(mgr, into);
@@ -45,7 +46,10 @@ void type_env::set_mangled_name(const std::string& name, const std::string& mang
 
 const std::string& type_env::get_mangled_name(const std::string& name) const {
     auto it = names.find(name);
-    if(it != names.end()) return it->second.mangled_name;
+    if(it != names.end()) {
+        assert(it->second.mangled_name);
+        return *it->second.mangled_name;
+    }
     assert(parent != nullptr);
     return parent->get_mangled_name(name);
 }
@@ -59,7 +63,7 @@ type_ptr type_env::lookup_type(const std::string& name) const {
 
 void type_env::bind(const std::string& name, type_ptr t, visibility v) {
     type_scheme_ptr new_scheme(new type_scheme(std::move(t)));
-    names[name] = variable_data(std::move(new_scheme), v, "");
+    names[name] = variable_data(std::move(new_scheme), v, std::nullopt);
 }
 
 void type_env::bind(const std::string& name, type_scheme_ptr t, visibility v) {
diff --git a/code/compiler/13/type_env.hpp b/code/compiler/13/type_env.hpp
index f5d98ed..2bc3450 100644
--- a/code/compiler/13/type_env.hpp
+++ b/code/compiler/13/type_env.hpp
@@ -2,6 +2,7 @@
 #include <map>
 #include <string>
 #include <set>
+#include <optional>
 #include "graph.hpp"
 #include "type.hpp"
 
@@ -15,11 +16,11 @@ class type_env {
         struct variable_data {
             type_scheme_ptr type;
             visibility vis;
-            std::string mangled_name;
+            std::optional<std::string> mangled_name;
 
             variable_data()
-                : variable_data(nullptr, visibility::local, "") {}
-            variable_data(type_scheme_ptr t, visibility v, std::string n)
+                : variable_data(nullptr, visibility::local, std::nullopt) {}
+            variable_data(type_scheme_ptr t, visibility v, std::optional<std::string> n)
                 : type(std::move(t)), vis(v), mangled_name(std::move(n)) {}
         };