Status command doesn't support 'X' for svn:externals

I've been trying to make a release by running 'mvn release:prepare', but got a warning saying that 'X' is an unknown file status.
I noticed the release plugin runs the 'svn --non-interactive status' command internally which prints the status of working copy files and directories. Since one of the directories was fetched from a different svn repo thru svn:externals, its status is 'X' and it does not seem to be recognized by the release plugin. Therefore the outcome of 'mvn release:prepare' command is a build failure.