diff --git a/src/field_instance.h b/src/field_instance.h
index eb72e78..44d1bce 100644
--- a/src/field_instance.h
+++ b/src/field_instance.h
@@ -414,9 +414,9 @@
         return static_cast<const Fn*>(this)
             ->template ForType<std::unique_ptr<protobuf::Message>>(field,
                                                                    args...);
-      default:
-        assert(!"Unknown type");
     }
+    assert(!"Unknown type");
+    abort();
   }
 };