Setting maven.javadoc.isoffline seems to have no effect

I'm trying to work around MJAVADOC-641 and the proxy issues that we are having, and I'd like to set isOffline to true.     I tried to do this my setting -Dmaven.javadoc.isOffline=true in my maven invocation, and it doesn't seem to take effect.    Setting isOffline to true within the <configuration/> allows my build to succeed.

Am I using the wrong command line property for isOffline?