Time Conditions cron task output not redirected to /dev/null

[root@freepbx modules]# crontab -l -u asterisk | grep schedtc
* * * * * [ -x /var/lib/asterisk/bin/schedtc.php ] && /var/lib/asterisk/bin/schedtc.php

If the script generates a whoops error or any other output, an email is triggered once a minute.