Invalid value in RESP column (case 4276)

I am using the 2008 version of DirectRT for running the Stroop task. Participants' responses are audio recorded. I am confused when looking at my output files. Although participants are not required to press any key, there is a -1 in the RESP column. We also use the 2012 version for the same task which indicates a zero in the same column. Is this something I should pay attention to? I have read a few other posts which stated that this is just a bug in older versions.

Thanks!

Can you upload a copy of your input file and output files?