diff --git a/tools/testing/selftests/dt/ktap_helpers.sh b/tools/testing/selftests/dt/ktap_helpers.sh
index 8dfae51bb4e2..9faa892a8845 100644
--- a/tools/testing/selftests/dt/ktap_helpers.sh
+++ b/tools/testing/selftests/dt/ktap_helpers.sh
@@ -27,7 +27,7 @@ ktap_skip_all() {
 __ktap_test() {
 	result="$1"
 	description="$2"
-	directive="$3" # optional
+	directive="${3:-}" # optional
 
 	local directive_str=
 	[[ ! -z "$directive" ]] && directive_str="# $directive"