diff --git a/t/t6026-merge-attr.sh b/t/t6026-merge-attr.sh
index 7a6e33e673..8f9b48a493 100755
--- a/t/t6026-merge-attr.sh
+++ b/t/t6026-merge-attr.sh
@@ -183,16 +183,24 @@ test_expect_success 'up-to-date merge without common ancestor' '
 
 test_expect_success 'custom merge does not lock index' '
 	git reset --hard anchor &&
-	write_script sleep-one-second.sh <<-\EOF &&
-		sleep 1 &
+	write_script sleep-an-hour.sh <<-\EOF &&
+		sleep 3600 &
 		echo $! >sleep.pid
 	EOF
-	test_when_finished "kill \$(cat sleep.pid)" &&
 
 	test_write_lines >.gitattributes \
-		"* merge=ours" "text merge=sleep-one-second" &&
+		"* merge=ours" "text merge=sleep-an-hour" &&
 	test_config merge.ours.driver true &&
-	test_config merge.sleep-one-second.driver ./sleep-one-second.sh &&
+	test_config merge.sleep-an-hour.driver ./sleep-an-hour.sh &&
+
+	# We are testing that the custom merge driver does not block
+	# index.lock on Windows due to an inherited file handle.
+	# To ensure that the backgrounded process ran sufficiently
+	# long (and has been started in the first place), we do not
+	# ignore the result of the kill command.
+	# By packaging the command in test_when_finished, we get both
+	# the correctness check and the clean-up.
+	test_when_finished "kill \$(cat sleep.pid)" &&
 	git merge master
 '