Unary negation failure with TemporalPredicate and declared timestamp

declare TransactionHistory
  @role( event )
  @timestamp(timeStamp)
end
rule R
when
    $a : TransactionHistory( merchantId == 10 )
   List(size() > 0) from collect (TransactionHistory( merchantId == 20, !(this before [10d] $a.timeStamp )))
then
...