diff --git a/example/pl0.cc b/example/pl0.cc
index e94ae21..3b7b7ff 100644
--- a/example/pl0.cc
+++ b/example/pl0.cc
@@ -244,6 +244,7 @@ private:
             case "<"_:  return lval < rval;
             case ">="_: return lval >= rval;
             case ">"_:  return lval > rval;
+            default:    throw logic_error("invalid operator");
         }
     }