path handling problems in copy

I have a sandbox which is a checked-out trunk directory; the parent
directory is not versioned. In order to tag the sandbox, I do

svn copy . <repo-url>/tags/another

This gives

svn: Trying to use an unsupported feature
 svn: cannot copy path '' into its own child  '<repo-url>/tags/another'

I don't think I'm trying to copy a path into its own child here.

Original issue reported by loewis