diff --git a/lib/ngtcp2_err.c b/lib/ngtcp2_err.c
index 58f9671e1eaca0d88ea0ef20d3c87553585e4c7e..071b86ab40c922878ca87331e4088081cead4801 100644
--- a/lib/ngtcp2_err.c
+++ b/lib/ngtcp2_err.c
@@ -97,7 +97,7 @@ const char *ngtcp2_strerror(int liberr) {
   case NGTCP2_ERR_INTERNAL:
     return "ERR_INTERNAL";
   default:
-    return "UNKNOWN";
+    return "(unknown)";
   }
 }