diff --git a/code/compiler/13/ast.cpp b/code/compiler/13/ast.cpp
index a59e995..8a99486 100644
--- a/code/compiler/13/ast.cpp
+++ b/code/compiler/13/ast.cpp
@@ -43,7 +43,10 @@ void ast_lid::find_free(std::set<std::string>& into) {
 
 type_ptr ast_lid::typecheck(type_mgr& mgr, type_env_ptr& env) {
     this->env = env;
-    return env->lookup(id)->instantiate(mgr);
+    type_scheme_ptr lid_type = env->lookup(id);
+    if(!lid_type)
+        throw type_error(std::string("unknown identifier ") + id, loc);
+    return lid_type->instantiate(mgr);
 }
 
 void ast_lid::translate(global_scope& scope) {
@@ -69,7 +72,10 @@ void ast_uid::find_free(std::set<std::string>& into) {
 
 type_ptr ast_uid::typecheck(type_mgr& mgr, type_env_ptr& env) {
     this->env = env;
-    return env->lookup(id)->instantiate(mgr);
+    type_scheme_ptr uid_type = env->lookup(id);
+    if(!uid_type)
+        throw type_error(std::string("unknown constructor ") + id, loc);
+    return uid_type->instantiate(mgr);
 }
 
 void ast_uid::translate(global_scope& scope) {