scala.concurrent.Future{1} zip null

java.lang.IllegalStateException: problem in scala.concurrent internal callback
	at scala.concurrent.Future$InternalCallbackExecutor$.reportFailure(Future.scala:589)
	at scala.concurrent.impl.CallbackRunnable.executeWithValue(Promise.scala:40)
	at scala.concurrent.impl.Promise$DefaultPromise.scala$concurrent$impl$Promise$DefaultPromise$$dispatchOrAddCallback(Promise.scala:280)
	at scala.concurrent.impl.Promise$DefaultPromise.onComplete(Promise.scala:270)
	at scala.concurrent.Future$class.zip(Future.scala:361)
	at scala.concurrent.impl.Promise$DefaultPromise.zip(Promise.scala:153)
	at org.backuity.photo.IndexerTest.ff(IndexerTest.scala:34)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)
	at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
	at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)
	at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
	at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)
	at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)
	at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)
	at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)
	at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)
	at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)
	at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)
	at org.junit.runners.ParentRunner.run(ParentRunner.java:300)
	at org.junit.runner.JUnitCore.run(JUnitCore.java:157)
	at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:74)
	at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:202)
	at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:65)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: java.lang.IllegalArgumentException: requirement failed
	at scala.Predef$.require(Predef.scala:221)
	at scala.concurrent.Future$InternalCallbackExecutor$Batch.run(Future.scala:628)
	at scala.concurrent.Future$InternalCallbackExecutor$.scala$concurrent$Future$InternalCallbackExecutor$$unbatchedExecute(Future.scala:691)
	at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.processBatch$1(Future.scala:649)
	at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.apply$mcV$sp(Future.scala:655)
	at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.apply(Future.scala:632)
	at scala.concurrent.Future$InternalCallbackExecutor$Batch$$anonfun$run$1.apply(Future.scala:632)
	at scala.concurrent.BlockContext$.withBlockContext(BlockContext.scala:72)
	at scala.concurrent.Future$InternalCallbackExecutor$Batch.run(Future.scala:631)
	at scala.concurrent.Future$InternalCallbackExecutor$.scala$concurrent$Future$InternalCallbackExecutor$$unbatchedExecute(Future.scala:691)
	at scala.concurrent.Future$InternalCallbackExecutor$.execute(Future.scala:682)
	... 32 more

 case t: Throwable =>
                    // if one task throws, move the
                    // remaining tasks to another thread
                    // so we can throw the exception
                    // up to the invoking executor
                    val remaining = _tasksLocal.get
                    _tasksLocal set Nil
                    unbatchedExecute(new Batch(remaining))
                    throw t