What's going on here?

jencode.md

import argonaut._, Argonaut._
import scala.util.control.NoStackTrace

final case class UncaughtException(error: String) extends RuntimeException with NoStackTrace

implicit val UncaughtExceptionJsonEncoder: EncodeJson[UncaughtException] =
  jencode1L((e: UncaughtException) => e.getMessage)("error")

scala> UncaughtExceptionJsonEncoder.encode(UncaughtException("a"))
java.lang.NullPointerException
  at scala.collection.immutable.StringOps$.length$extension(StringOps.scala:47)
  at scala.collection.immutable.StringOps.length(StringOps.scala:47)

but the following works:

    EncodeJson((p: UncaughtException) =>
      ("error" := p.error) ->: jEmptyObject)

scala> X.encode(UncaughtException("a"))
res1: argonaut.Json = {"error":"a"}```