How to compute _indextime-_time difference average with tstats?

Hi, I'd like to calculate the average latency (_indextime-_time) with the tstats command, but I can not make it work: | tstats avg(_indextime-_time) where (index=* OR index=_*) by index Splunk thinks "_indextime-_time" is a field name. How can I compute the difference in the tstats? Thank you