$ cat CollectToMap.java
import java.util.Collections;
import java.util.stream.Collectors;

public class CollectToMap {

    public static void main(String[] args) {
        Collections.singleton(null)
                   .stream()
                   .collect(Collectors.toMap(k -> new Object(), v -> null));
    }
}
$ javac CollectToMap.java
$ java CollectToMap
Exception in thread "main" java.lang.NullPointerException
        at java.base/java.util.Objects.requireNonNull(Objects.java:208)
        at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:180)
        at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
        at java.base/java.util.Collections$2.tryAdvance(Collections.java:4820)
        at java.base/java.util.Collections$2.forEachRemaining(Collections.java:4828)
        at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
        at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
        at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
        at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
        at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:682)
        at CollectToMap.main(CollectToMap.java:9)

----------------------
This is unexpected because neither Stream.collect nor Collectors.toMap mentions this. This is also surprising as null key is okay:

$ cat CollectToMap.java
import java.util.Collections;
import java.util.stream.Collectors;

public class CollectToMap {

    public static void main(String[] args) {
        Collections.singleton(null)
                   .stream()
                   .collect(Collectors.toMap(k -> null, v -> new Object()));
    }
}
$ javac CollectToMap.java
$ java CollectToMap
$