-u flag not working?

I am trying to uninstall broken versions of some ports. But I get this: [Instance/bin] > uninstall -u libxml2 ---> The following versions of libxml2 are currently installed: ---> libxml2 @2.6.28_0 ---> libxml2 @2.6.28_1 (active) ---> libxml2 @2.6.28_2 Error: port uninstall failed: Registry error: Please specify the full version as recorded in the port registry. I thought the -u flag would uninstall non-active versions of a port? ET