Setting 'testFrameworks' in build.sbt of a scalajs project causes java.lang.NullPointerException when running test

I am new to scala and scalajs. I am trying to add unit tests to my scalajs project using minitest. I followed instructions at http://ift.tt/2FMG11Q, so this is what my build.sbt looks like:

enablePlugins(ScalaJSPlugin, WorkbenchPlugin)
name := "Test"
version := "0.1-SNAPSHOT"
scalaVersion := "2.11.8"
libraryDependencies ++= Seq(
  "org.scala-js" %%% "scalajs-dom" % "0.9.1",
  "com.lihaoyi" %%% "scalatags" % "0.6.1",
  "io.monix" %%% "minitest" % "2.0.0" % "test"
)
testFrameworks += new TestFramework("minitest.runner.Framework")

I also have a project/build.sbt that looks like:

addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.17")
addSbtPlugin("com.lihaoyi" % "workbench" % "0.3.0")

When I run:

> sbt test

I see this error:

java.lang.NullPointerException
at scala.collection.convert.Wrappers$JMapWrapperLike$$anon$2.<init>(Wrappers.scala:265)
at scala.collection.convert.Wrappers$JMapWrapperLike$class.iterator(Wrappers.scala:264)
at scala.collection.convert.Wrappers$JMapWrapper.iterator(Wrappers.scala:275)
at scala.collection.IterableLike$class.foreach(IterableLike.scala:72)
at scala.collection.AbstractIterable.foreach(Iterable.scala:54)
at scala.collection.TraversableOnce$class.toMap(TraversableOnce.scala:279)
at scala.collection.AbstractTraversable.toMap(Traversable.scala:105)
at org.scalajs.core.tools.json.Impl$.toMap(Impl.scala:26)
at org.scalajs.core.tools.json.JSONObjExtractor.<init>(JSONObjExtractor.scala:6)
at org.scalajs.testadapter.FrameworkInfo$Deserializer$.deserialize(FrameworkInfo.scala:24)
at org.scalajs.testadapter.FrameworkInfo$Deserializer$.deserialize(FrameworkInfo.scala:22)
at org.scalajs.core.tools.json.package$.fromJSON(package.scala:19)
at org.scalajs.testadapter.ScalaJSFramework.fetchFrameworkInfo(ScalaJSFramework.scala:66)
at org.scalajs.testadapter.ScalaJSFramework.<init>(ScalaJSFramework.scala:37)
at org.scalajs.sbtplugin.ScalaJSPluginInternal$$anonfun$76$$anonfun$apply$52.apply(ScalaJSPluginInternal.scala:873)
at org.scalajs.sbtplugin.ScalaJSPluginInternal$$anonfun$76$$anonfun$apply$52.apply(ScalaJSPluginInternal.scala:872)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:244)
at scala.collection.immutable.Map$Map1.foreach(Map.scala:109)
at scala.collection.TraversableLike$class.map(TraversableLike.scala:244)
at scala.collection.AbstractTraversable.map(Traversable.scala:105)
at org.scalajs.sbtplugin.ScalaJSPluginInternal$$anonfun$76.apply(ScalaJSPluginInternal.scala:872)
at org.scalajs.sbtplugin.ScalaJSPluginInternal$$anonfun$76.apply(ScalaJSPluginInternal.scala:849)
...

I don't see the crash if I remove this line from build.sbt, but then of course my tests don't get run:

testFrameworks += new TestFramework("minitest.runner.Framework")

The problem doesn't seem specific to minitest because I see it with otest as well. I'm not sure what I should try next to resolve this problem, so I'd appreciate any help with it.