PHP 7.1 void返回类型的理解

Question

I have done some tests with the next minor Version of PHP (7.1beta).

During the tests with void return types I noticed that gettype returns NULL for a void method. Can anyone explain why it does that?

function returnVoid(): void {
    return;
}

echo gettype(returnVoid()); // returns "NULL"

Is this the expected behavior?