diff --git a/lib/isc/assertions.c b/lib/isc/assertions.c
index 21ca21b638..b5309bb2bb 100644
--- a/lib/isc/assertions.c
+++ b/lib/isc/assertions.c
@@ -82,7 +82,7 @@ isc_assertion_typetotext(isc_assertiontype_t type) {
 		result = "INVARIANT";
 		break;
 	default:
-		result = NULL;
+		result = "UNKNOWN";
 	}
 	return (result);
 }