Less compiler error handling is broken

This is with 2.2.0-M2

    sbt.PlayExceptions$UnexpectedException: Unexpected exception[ClassCastException: org.mozilla.javascript.ConsString cannot be cast to java.lang.String]
    	at sbt.PlayReloader$$anon$2$$anonfun$reload$2$$anonfun$apply$14.apply(PlayReloader.scala:323) ~[na:na]
    	at sbt.PlayReloader$$anon$2$$anonfun$reload$2$$anonfun$apply$14.apply(PlayReloader.scala:312) ~[na:na]
    	at scala.Option.map(Option.scala:145) ~[scala-library.jar:na]
    	at sbt.PlayReloader$$anon$2$$anonfun$reload$2.apply(PlayReloader.scala:312) ~[na:na]
    	at sbt.PlayReloader$$anon$2$$anonfun$reload$2.apply(PlayReloader.scala:310) ~[na:na]
    	at scala.util.Either$LeftProjection.map(Either.scala:377) ~[scala-library.jar:na]
    java.lang.ClassCastException: org.mozilla.javascript.ConsString cannot be cast to java.lang.String
    	at play.core.less.LessCompiler$.compile(LessCompiler.scala:140) ~[na:na]
    	at sbt.PlayAssetsCompiler$$anonfun$6.apply(PlayAssetsCompiler.scala:88) ~[na:na]
    	at sbt.PlayAssetsCompiler$$anonfun$6.apply(PlayAssetsCompiler.scala:88) ~[na:na]
    	at sbt.PlayAssetsCompiler$$anonfun$AssetsCompiler$1$$anonfun$3.liftedTree1$1(PlayAssetsCompiler.scala:52) ~[na:na]
    	at sbt.PlayAssetsCompiler$$anonfun$AssetsCompiler$1$$anonfun$3.apply(PlayAssetsCompiler.scala:51) ~[na:na]
    	at sbt.PlayAssetsCompiler$$anonfun$AssetsCompiler$1$$anonfun$3.apply(PlayAssetsCompiler.scala:48) ~[na:na]