diff --git a/11/type.cpp b/11/type.cpp
index 8022966..05e85c6 100644
--- a/11/type.cpp
+++ b/11/type.cpp
@@ -114,7 +114,7 @@ void type_mgr::unify(type_ptr l, type_ptr r) {
         return;
     } else if((lid = dynamic_cast<type_base*>(l.get())) &&
             (rid = dynamic_cast<type_base*>(r.get()))) {
-        if(lid->name == rid->name) return;
+        if(lid->name == rid->name && lid->arity == rid->arity) return;
     } else if((lapp = dynamic_cast<type_app*>(l.get())) &&
             (rapp = dynamic_cast<type_app*>(r.get()))) {
         unify(lapp->constructor, rapp->constructor);