FULL PRODUCT VERSION :
java version "1.8.0_72"
Java(TM) SE Runtime Environment (build 1.8.0_72-b15)
Java HotSpot(TM) 64-Bit Server VM (build 25.72-b15, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 6.3.9600]

A DESCRIPTION OF THE PROBLEM :
When used on null values, Collectors.toMap fails with a NullPointerException, see source below

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Look at the source code below: the first collect operation works as expected, the second crashes.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Both collect operations should work
ACTUAL -
Exception in thread "main" java.lang.NullPointerException
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at java.util.concurrent.ForkJoinTask.getThrowableException(ForkJoinTask.java:598)
at java.util.concurrent.ForkJoinTask.reportException(ForkJoinTask.java:677)
at java.util.concurrent.ForkJoinTask.invoke(ForkJoinTask.java:735)
at java.util.stream.ReduceOps$ReduceOp.evaluateParallel(ReduceOps.java:714)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:233)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)
at stuff.TestMapCollectorProblem.main(TestMapCollectorProblem.java:15)
Caused by: java.lang.NullPointerException
at java.util.HashMap.merge(HashMap.java:1216)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1683)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:747)
at java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:721)
at java.util.stream.AbstractTask.compute(AbstractTask.java:316)
at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)


REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
package stuff;

import java.util.HashMap;
import java.util.stream.Collectors;

public class TestMapCollectorProblem {
    public static void main(String[] args) {
        HashMap<String, String> m = new HashMap<>();
        m.put("A", "a");
        m.put("B", "b");
        m.put("C", "c");
        m.entrySet().parallelStream().collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));

        m.put("D", null);
        m.entrySet().parallelStream().collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue()));
    }
}
---------- END SOURCE ----------