segfault when trying to use nrows on update query return

[gmcalist]

I'm building a custom benchmark where I would like to be able to understand the number of rows updated by a particular statement so that I can do particular logic based on that. I have tried the same technique that I've used on my select statements but when I attempt it for an update statement I get a segafult. Here is a partial snippet of what I trying with PostgreSQL.

rs=con:query(([[UPDATE ...
if rs.nrows == 0 then

The segfault only happens if I try to use rs.nrows not the actual query execution.