diff --git a/tests/sysfs_edid_timing b/tests/sysfs_edid_timing
index 22340cba..3a8c6c0e 100755
--- a/tests/sysfs_edid_timing
+++ b/tests/sysfs_edid_timing
@@ -10,7 +10,7 @@ cat $(find /sys/devices/|grep drm | grep /status) > /dev/null
 TIME2=$(date +%s%N)
 
 # time in ms
-RES=$[(TIME2 - TIME1) / 1000000]
+RES=$(((TIME2 - TIME1) / 1000000))
 
 if [ $RES -gt 600 ]; then
 	echo "Talking to outputs took ${RES}ms, something is wrong"