diff --git a/12/ast.cpp b/12/ast.cpp
index df996dc..09fece2 100644
--- a/12/ast.cpp
+++ b/12/ast.cpp
@@ -355,10 +355,11 @@ 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_ptr constructor_type = env->lookup(constr)->instantiate(mgr);
-    if(!constructor_type) {
+    type_scheme_ptr constructor_type_scheme = env->lookup(constr);
+    if(!constructor_type_scheme) {
         throw type_error(std::string("pattern using unknown constructor ") + constr);
     }
+    type_ptr constructor_type = constructor_type_scheme->instantiate(mgr);
 
     for(auto& param : params) {
         type_arr* arr = dynamic_cast<type_arr*>(constructor_type.get());