EC2: "Groups belonging to other users may not be granted permissions to VPC security groups."

EC2: "Groups belonging to other users may not be granted permissions to
VPC security groups."

I am deploying to EC2 using rubber (following Railscast #347). I have just
run "cap rubber:create_staging" and am getting this error:
"Groups belonging to other users may not be granted permissions to VPC
security groups."
This is my first contact with either rubber or EC2 so this means very
little to me. Can anyone point me in the right direction?