diff --git a/tests/bin/prepare-wp-upstreams.sh b/tests/bin/prepare-wp-upstreams.sh
index 963b4277..edc61c4d 100755
--- a/tests/bin/prepare-wp-upstreams.sh
+++ b/tests/bin/prepare-wp-upstreams.sh
@@ -58,7 +58,7 @@ done
 function update_json() {
 	echo "Checking diff between '${working_copy_path}/wp' and '${working_copy_path}/wpms'..."
 	diff=$(diff "${working_copy_path}/wp" "${working_copy_path}/wpms")
-	if [ $diff > /dev/null ]; then
+	if [ -n $diff ]; then
 		echo "wp fixture is already ahead of wpms fixture. Skipping update."
 	else
 		# Parse the JSON file.