Fragment Evaluation Error

Question

Can someone tell me what "Fragment evaluation error" means, or where I might look for solutions? I sometimes (but not always) get lots of these errors (without changing my code):

[error] ! Fragment evaluation error
[error]     ThrowableException: Could not initialize class code.model.Post$  (FutureTask.java:138)
[error] code.model.PostSpec$$anonfun$1$$anonfun$apply$1.apply$mcZ$sp(PostSpec.scala:68)
[error] code.model.PostSpec$$anonfun$1$$anonfun$apply$1.apply(PostSpec.scala:51)
[error] code.model.PostSpec$$anonfun$1$$anonfun$apply$1.apply(PostSpec.scala:51)

Line 68 of PostSpec is the first line in the (specs2) test that references the Post model companion object:

val test4 = Post.fixJValue(toextract4).extract[Selection]

I'm using Scala 2.9.0-1.

Also: I have no idea whether it matters, but Post is a net.liftweb.mongodb.record.MongoRecord class companion object:

object Post extends Post with MongoMetaRecord[Post] { ... }