Mysterious "unknown constant: 18" error with Scala 2.11 and Java 8

Hi,

I get this error now, trying to compile my Scala/Java project in Java8 and Scala 2.11.  I have no idea what it means, and there is no line number, or file, so some component (looks like maybe sbt?) has a bug in it.  Anyone know what this is or how to fix it?  I flushed caches, restarted, rebuilt project.  I'm using IDEA 13.

Rob

scala: Error: Unknown constant: 18

java.lang.RuntimeException: Unknown constant: 18

at scala.sys.package$.error(package.scala:27)

at sbt.classfile.Parser$.getConstant(Parser.scala:150)

at sbt.classfile.Parser$.parse$1(Parser.scala:127)

at sbt.classfile.Parser$.sbt$classfile$Parser$$parseConstantPool(Parser.scala:132)

at sbt.classfile.Parser$$anon$1.<init>(Parser.scala:32)

at sbt.classfile.Parser$.parseImpl(Parser.scala:24)

at sbt.classfile.Parser$.sbt$classfile$Parser$$parse(Parser.scala:20)

at sbt.classfile.Parser$$anonfun$apply$1.apply(Parser.scala:19)

at sbt.classfile.Parser$$anonfun$apply$1.apply(Parser.scala:19)

at sbt.Using.apply(Using.scala:25)

at sbt.classfile.Parser$.apply(Parser.scala:19)

at sbt.classfile.Analyze$$anonfun$apply$2.apply(Analyze.scala:32)

at sbt.classfile.Analyze$$anonfun$apply$2.apply(Analyze.scala:31)

at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)

at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)

at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)

at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:47)

at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)

at scala.collection.AbstractTraversable.map(Traversable.scala:105)

at sbt.classfile.Analyze$.apply(Analyze.scala:31)

at sbt.compiler.AggressiveCompile$$anonfun$6$$anonfun$compileJava$1$2$$anonfun$apply$mcV$sp$2.apply(AggressiveCompile.scala:147)

at sbt.compiler.AggressiveCompile$$anonfun$6$$anonfun$compileJava$1$2$$anonfun$apply$mcV$sp$2.apply(AggressiveCompile.scala:145)

at scala.collection.TraversableLike$WithFilter$$anonfun$foreach$1.apply(TraversableLike.scala:772)

at scala.collection.immutable.List.foreach(List.scala:309)

at scala.collection.TraversableLike$WithFilter.foreach(TraversableLike.scala:771)

at sbt.compiler.AggressiveCompile$$anonfun$6$$anonfun$compileJava$1$2.apply$mcV$sp(AggressiveCompile.scala:145)

at sbt.compiler.AggressiveCompile$$anonfun$6$$anonfun$compileJava$1$2.apply(AggressiveCompile.scala:145)

at sbt.compiler.AggressiveCompile$$anonfun$6$$anonfun$compileJava$1$2.apply(AggressiveCompile.scala:145)

at sbt.compiler.AggressiveCompile.sbt$compiler$AggressiveCompile$$timed(AggressiveCompile.scala:173)

at sbt.compiler.AggressiveCompile$$anonfun$6.compileJava$1(AggressiveCompile.scala:144)

at sbt.compiler.AggressiveCompile$$anonfun$6.apply(AggressiveCompile.scala:151)

at sbt.compiler.AggressiveCompile$$anonfun$6.apply(AggressiveCompile.scala:89)

at sbt.inc.IncrementalCompile$$anonfun$doCompile$1.apply(Compile.scala:39)

at sbt.inc.IncrementalCompile$$anonfun$doCompile$1.apply(Compile.scala:37)

at sbt.inc.Incremental$.cycle(Incremental.scala:75)

at sbt.inc.Incremental$$anonfun$1.apply(Incremental.scala:34)

at sbt.inc.Incremental$$anonfun$1.apply(Incremental.scala:33)

at sbt.inc.Incremental$.manageClassfiles(Incremental.scala:42)

at sbt.inc.Incremental$.compile(Incremental.scala:33)

at sbt.inc.IncrementalCompile$.apply(Compile.scala:27)

at sbt.compiler.AggressiveCompile.compile2(AggressiveCompile.scala:164)

at sbt.compiler.AggressiveCompile.compile1(AggressiveCompile.scala:73)

at org.jetbrains.jps.incremental.scala.local.CompilerImpl.compile(CompilerImpl.scala:61)

at org.jetbrains.jps.incremental.scala.local.LocalServer.compile(LocalServer.scala:26)

at org.jetbrains.jps.incremental.scala.remote.Main$.nailMain(Main.scala:31)

at org.jetbrains.jps.incremental.scala.remote.Main.nailMain(Main.scala)

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)

at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)

at java.lang.reflect.Method.invoke(Method.java:601)

at com.martiansoftware.nailgun.NGSession.run(Unknown Source)