diff --git a/code/compiler/13/ast.cpp b/code/compiler/13/ast.cpp
index 5153fa4..70c5446 100644
--- a/code/compiler/13/ast.cpp
+++ b/code/compiler/13/ast.cpp
@@ -47,7 +47,7 @@ type_ptr ast_lid::typecheck(type_mgr& mgr, type_env_ptr& env) {
     this->env = env;
     type_scheme_ptr lid_type = env->lookup(id);
     if(!lid_type)
-        throw type_error(std::string("unknown identifier ") + id, loc);
+        throw type_error("unknown identifier " + id, loc);
     return lid_type->instantiate(mgr);
 }
 
@@ -75,7 +75,7 @@ type_ptr ast_uid::typecheck(type_mgr& mgr, type_env_ptr& env) {
     this->env = env;
     type_scheme_ptr uid_type = env->lookup(id);
     if(!uid_type)
-        throw type_error(std::string("unknown constructor ") + id, loc);
+        throw type_error("unknown constructor " + id, loc);
     return uid_type->instantiate(mgr);
 }
 
@@ -105,7 +105,7 @@ type_ptr ast_binop::typecheck(type_mgr& mgr, type_env_ptr& env) {
     type_ptr ltype = left->typecheck(mgr, env);
     type_ptr rtype = right->typecheck(mgr, env);
     type_ptr ftype = env->lookup(op_name(op))->instantiate(mgr);
-    if(!ftype) throw type_error(std::string("unknown binary operator ") + op_name(op), loc);
+    if(!ftype) throw type_error("unknown binary operator " + op_name(op), loc);
 
     // For better type errors, we first require binary function,
     // and only then unify each argument. This way, we can
@@ -438,7 +438,7 @@ void pattern_constr::find_variables(std::set<std::string>& into) const {
 void pattern_constr::typecheck(type_ptr t, type_mgr& mgr, type_env_ptr& env) const {
     type_scheme_ptr constructor_type_scheme = env->lookup(constr);
     if(!constructor_type_scheme) {
-        throw type_error(std::string("pattern using unknown constructor ") + constr, loc);
+        throw type_error("pattern using unknown constructor " + constr, loc);
     }
     type_ptr constructor_type = constructor_type_scheme->instantiate(mgr);