svn checkout --set-depth ignores externals

Hi,

In the root of our trunk, we have a 'build_tools' folder which is defined
as an external.

For example
/trunk
    /build_tools (external)
    /alpha
        /A
    /beta
    /gamma
    build.xml
    build.properties

If we checkout using 'svn checkout --depth immediates', this folder is
ignored, but I continue to get alpha/beta/gamma folders (empty). I can't
seem to get it back without checking out the entirety of trunk.

Any ideas? Is this intended behaviour?

---
Daniel Becroft