Error Unable to get fields from previous steps because of an error

When i run my Transformation i have this error:

org.pentaho.di.trans.steps.groupby.GroupByMeta@1b3294c - ERROR (version 7.0.0.0-25, build 1 from 2016-11-05 15.35.36 by buildguy) : Unable to get fields from previous steps because of an error

the running go good but i have this error in console?
WHY?
what is?
I have in input 1 TABLE INPUT with a POSTGRES QUERY with two variable ${VAR1} and ${VAR2}. Can be this a problem? for me is all ok..

thanks