UnknownDescriptionException: Descr ( class org.drools.lang.descr.MVELExprDescr ) is unknown to drools verifier.

When the verifier encounters a rule such as:

rule "Contrived Example"
when
$pitem : PresentedItem(sequenceNumber > 0)
$receipt : Receipt(items contains $pitem.id)
$item : ReceiptItem(plan == foo.Plan.NULL_PLAN) from $receipt.items
then
log.info("Contrived Example ran");
end

It prints this exception to standard error and swallows the exception (as seen in VerifierImpl.java line 93):

org.drools.verifier.visitor.UnknownDescriptionException: Descr ( class org.drools.lang.descr.MVELExprDescr ) is unknown to drools verifier.
at org.drools.verifier.visitor.DeclarativeInvokerDescrVisitor.visit(DeclarativeInvokerDescrVisitor.java:58)
at org.drools.verifier.visitor.PatternDescrVisitor.visit(PatternDescrVisitor.java:297)
...

It seems to be choking on the token $receipt.items in the rule. Not sure if this is a bug or an accidental bit of debug code using stack traces that snuck into a release. I'd like to know which, at least. Thank you.