configSource doesn’t work in system.serviceModel *or* its subsections

VS.NET’s editor moans about the config, but it works.

I have config like this…

<system.serviceModel>
  <behaviors configSource="config\system.servicemodel.behaviors.config" />
  <bindings configSource="config\system.servicemodel.bindings.config" />
  <client configSource="config\system.servicemodel.client.config" />
</system.serviceModel>

… which works fine.