problem with installing ros on mac osx lion (common_rosdeps)

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I am using homebrew

when installing ros with,

rosinstall ~/ros "http://packages.ros.org/cgi-bin/gen_rosinstall.py?rosdistro=electric&variant=desktop-full&overlay=no"

I got the following error.

Installing https://kforge.ros.org/common/rosdepcore common_rosdeps-1.0.2 to /Users/ze/ros/common_rosdeps
warning: kforge.ros.org certificate with fingerprint 9b:29:7b:22:3a:35:9c:e6:6c:ca:fe:b7:64:eb:1e:cd:bf:01:0a:92 not verified (check hostfingerprints or web.cacerts config setting)
pulling from https://kforge.ros.org/common/rosdepcore
searching for changes
no changes found
ERROR: Failed to install tree '/Users/ze/ros/common_rosdeps'
Update Failed of /Users/ze/ros/common_rosdeps

It seems to have something to do with 'common_rosdeps'. But I could not find any answer on the internet.

Any idea what to do?

Thanks