Object::getClass method reference resulting in methodref.return.invalid

This may be down to my ignorance of how generics work. But at first sight I think this code isn't wrong:

import java.util.function.Function;

import org.checkerframework.checker.nullness.qual.Nullable;

public class Packet<T> {

    public static <T> Packet<T> get(T object) {
        return new Packet<>(object);
    }

    private final @Nullable T value;

    private Packet(@Nullable T value) {
        this.value = value;
    }

    public <U> @Nullable U map(Function<T, @Nullable U> mappingFunction) {
        if (value == null) {
            return (U) null;
        }
        return mappingFunction.apply(value);
    }

    public static void main(String[] args) {
        Object obj = 2.14;
        System.out.println("class: "
                + Packet.get(obj).map(Object::getClass)
        );
    }
}

Yet it produces the following:

Packet.java:28: warning: [methodref.return.invalid] Incompatible return type
                + Packet.get(obj).map(Object::getClass)
                                      ^
  Method
    @Initialized @NonNull Class<? extends @Initialized @Nullable Object> getClass(@UnknownInitialization @NonNull Object this) in java.lang.Object
  is not a valid method reference for
    @Initialized @Nullable Class<? extends @Initialized @NonNull Object> apply(@Initialized @NonNull Function<@Initialized @NonNull Object, @Initialized @Nullable Class<? extends @Initialized @NonNull Object>> this, @Initialized @NonNull Object p0) in java.util.function.Function
  found   : @Initialized @NonNull Class<? extends @Initialized @Nullable Object>
  required: @Initialized @Nullable Class<? extends @Initialized @NonNull Object>

What does this mean?

Start by reproducing the warning from the Packet.java example with the nullness checker, then trace the handling of Object::getClass method references and the reported generic return types. Compare the inferred nullable annotations with the Function target type; done means explaining whether the diagnostic is valid or identifying the checker behavior that needs correction.