ScalaTest DeferredAbortedSuite error when running simple tests.

My original code had a lot more going on, which distracted me from the true cause of the problem. This captures the essential problem.

import org.scalatest.AsyncFlatSpec
import scala.concurrent.Future

class AsyncFlatSpecSpec extends AsyncFlatSpec
{
  it should "parse an XML file" in {
    // ... Parsing ...
    Future.successful(succeed)
  }

  it should "parse an XML file" in {
    // ... Serializing ...
    Future.successful(succeed)
  }
}

This produced these errors:

[info] DeferredAbortedSuite:
[error] Uncaught exception when running AsyncFlatSpecSpec: java.lang.ArrayIndexOutOfBoundsException: 17
[trace] Stack trace suppressed: run last test:testOnly for the full output.

There is no array access happening anywhere in my code. What's going on?

Running "last test:testOnly" wasn't much help:

[info] DeferredAbortedSuite:
[error] Uncaught exception when running AsyncFlatSpecSpec: java.lang.ArrayIndexOutOfBoundsException: 17
sbt.ForkMain$ForkError: java.lang.ArrayIndexOutOfBoundsException: 17
  at org.scalatest.exceptions.StackDepth$class.stackTraceElement(StackDepth.scala:63)
  at org.scalatest.exceptions.StackDepth$class.failedCodeFileName(StackDepth.scala:77)
  at org.scalatest.exceptions.StackDepthException.failedCodeFileName(StackDepthException.scala:36)
  at org.scalatest.exceptions.StackDepth$class.failedCodeFileNameAndLineNumberString(StackDepth.scala:59)
  at org.scalatest.exceptions.StackDepthException.failedCodeFileNameAndLineNumberString(StackDepthException.scala:36)
  at org.scalatest.tools.StringReporter$.withPossibleLineNumber(StringReporter.scala:442)
  at org.scalatest.tools.StringReporter$.stringsToPrintOnError(StringReporter.scala:916)
  at org.scalatest.tools.StringReporter$.fragmentsForEvent(StringReporter.scala:747)
  at org.scalatest.tools.Framework$SbtLogInfoReporter.apply(Framework.scala:622)
  at org.scalatest.tools.FilterReporter.apply(FilterReporter.scala:41)
  at org.scalatest.tools.SbtDispatchReporter$$anonfun$apply$1.apply(SbtDispatchReporter.scala:23)
  at org.scalatest.tools.SbtDispatchReporter$$anonfun$apply$1.apply(SbtDispatchReporter.scala:23)
  at scala.collection.Iterator$class.foreach(Iterator.scala:893)
  at scala.collection.AbstractIterator.foreach(Iterator.scala:1336)
  at scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
  at scala.collection.AbstractIterable.foreach(Iterable.scala:54)
  at org.scalatest.tools.SbtDispatchReporter.apply(SbtDispatchReporter.scala:23)
  at org.scalatest.tools.Framework$SbtReporter.apply(Framework.scala:1119)
  at org.scalatest.tools.Framework.org$scalatest$tools$Framework$$runSuite(Framework.scala:387)
  at org.scalatest.tools.Framework$ScalaTestTask.execute(Framework.scala:506)
  at sbt.ForkMain$Run$2.call(ForkMain.java:296)
  at sbt.ForkMain$Run$2.call(ForkMain.java:286)
  at java.util.concurrent.FutureTask.run(FutureTask.java:266)
  at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
  at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) 
  at java.lang.Thread.run(Thread.java:745)

Confused, I retreated to the non-Async version, to see if that fared any better.

import org.scalatest.FlatSpec
class FlatSpecSpec extends FlatSpec {

  it should "parse an XML file" in {
    // ... Parsing ...
    succeed
  }

  it should "parse an XML file" in {
    // ... Serializing ...
    succeed
  }

}

It produced this different, but still cryptic error message:

[info] DeferredAbortedSuite:
[info] Exception encountered when attempting to run a suite with class name: org.scalatest.DeferredAbortedSuite *** ABORTED *** (20 milliseconds)
[info]   Exception encountered when attempting to run a suite with class name: org.scalatest.DeferredAbortedSuite (AsyncFlatSpecSpec.scala:32)
[info] ScalaTest

For completeness, here are the related portions of my build.sbt:

scalaVersion := "2.11.8"
libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.0-M15" % "test"
libraryDependencies += "org.scalactic" %% "scalactic" % "3.0.0-M15"

This was ultimately a trivial mistake on my part, but I wanted to post this for the sake of anyone else Googling these errors.