if ($this->valid()) {
    $this->key++; // Previously below $this->resumeToken assignment
    $this->resumeToken = $this->extractResumeToken($this->csIt->current());
}

public function testResumeTokenNotFoundAdvancesKey()
{
    $pipeline =  [['$project' => ['_id' => 0 ]]];

    $operation = new Watch($this->manager, $this->getDatabaseName(), $this->getCollectionName(), $pipeline, $this->defaultOptions);
    $changeStream = $operation->execute($this->getPrimaryServer());

    /* Note: we intentionally do not start iteration with rewind() to ensure
     * that we test extraction functionality within next(). */
    $this->insertDocument(['x' => 1]);
    $this->insertDocument(['x' => 2]);
    $this->insertDocument(['x' => 3]);

    try {
        $changeStream->rewind();
        $this->fail('ResumeTokenException was not thrown');
    } catch (ResumeTokenException $e) {}

    $this->assertSame(0, $changeStream->key());

    try {
        $changeStream->next();
        $this->fail('ResumeTokenException was not thrown');
    } catch (ResumeTokenException $e) {}

    $this->assertSame(1, $changeStream->key());

    try {
        $changeStream->next();
        $this->fail('ResumeTokenException was not thrown');
    } catch (ResumeTokenException $e) {}

    $this->assertSame(2, $changeStream->key());
}