conflict with CLI option handling by optparse package

Hi

I made an R script that can take command line options with help from optparse package and Rscript.
However, when I add "-p" option for my own purpose, it seems to conflict with "future parallel processing" and I get following warning:
> Warning: future: Ignoring invalid number of processes specified in command-line option: -p XXXX
> Warning message:
> In parseCmdArgs() : NAs introduced by coercion

Is there any way to avoid this conflict other than using another option alphabet in optprase?
Thank you.