Problems setting def.scala.version with SBT plugin

If I had:


def.scala.version=2.10.1

in project/build.properties, I get:

 scala: Error: object scala.runtime in compiler mirror not found.
scala.reflect.internal.MissingRequirementError: object scala.runtime in compiler mirror not found.
 at scala.reflect.internal.MissingRequirementError$.signal(MissingRequirementError.scala:16)
 at scala.reflect.internal.MissingRequirementError$.notFound(MissingRequirementError.scala:17)
 at scala.reflect.internal.Mirrors$RootsBase.getModuleOrClass(Mirrors.scala:48)
 at scala.reflect.internal.Mirrors$RootsBase.getModuleOrClass(Mirrors.scala:40)
 at scala.reflect.internal.Mirrors$RootsBase.getModuleOrClass(Mirrors.scala:61)
 at scala.reflect.internal.Mirrors$RootsBase.getPackage(Mirrors.scala:172)
 at scala.reflect.internal.Mirrors$RootsBase.getRequiredPackage(Mirrors.scala:175)
 at scala.reflect.internal.Definitions$DefinitionsClass.RuntimePackage$lzycompute(Definitions.scala:181)
 at scala.reflect.internal.Definitions$DefinitionsClass.RuntimePackage(Definitions.scala:181)
 at scala.reflect.internal.Definitions$DefinitionsClass.RuntimePackageClass$lzycompute(Definitions.scala:182)
 at scala.reflect.internal.Definitions$DefinitionsClass.RuntimePackageClass(Definitions.scala:182)
 at scala.reflect.internal.Definitions$DefinitionsClass.AnnotationDefaultAttr$lzycompute(Definitions.scala:1015)
 at scala.reflect.internal.Definitions$DefinitionsClass.AnnotationDefaultAttr(Definitions


Removing that setting made it work again. But I would like to be able to control the version.