Sbt cannot find scala.reflect.Manifest when getting Unidoc.scala settings using scala 2.10.0

Question

I just upgraded my project to Scala 2.10.0 using SBT 0.12. But now, when running sbt, I get the following error:

java.lang.NoClassDefFoundError: scala/reflect/ManifestFactory$ at X.build.Unidoc$.<init>(Unidoc.scala:8) at X.build.Unidoc$.<clinit>(Unidoc.scala) at X.build.ServicesBuild$.<init>(Build.scala:25) at X.build.ServicesBuild$.<clinit>(Build.scala) 

There seems to be a problem on SettingKey :

 8: val unidocDirectory = SettingKey[File]("unidoc-directory") 

I heard that Scala 2.10 did a different reflection than before, and thought it was a problem, but the Akka project does the same, and (I suppose) they are all fine.

https://github.com/akka/akka/blob/master/project/Unidoc.scala (my Unidoc.scala is pretty much a copy and their paste).

Does anyone know what is going on? How can he not find a class in Scala himself?

Thanks!