diff --git a/tools/testing/selftests/power_supply/test_power_supply_properties.sh b/tools/testing/selftests/power_supply/test_power_supply_properties.sh
index df272dfe1d2a9d..a66b1313ed8824 100755
--- a/tools/testing/selftests/power_supply/test_power_supply_properties.sh
+++ b/tools/testing/selftests/power_supply/test_power_supply_properties.sh
@@ -23,7 +23,7 @@ count_tests() {
 	total_tests=0
 
 	for i in $SUPPLIES; do
-		total_tests=$(("$total_tests" + "$NUM_TESTS"))
+		total_tests=$((total_tests + NUM_TESTS))
 	done
 
 	echo "$total_tests"