QTextToSpeechProcessor volume member being int instead of double

I'm developing a plugin for Nuance Vocalizer TTS, I subclassed QTextToSpeechProcessor for that purpose, and I'm having trouble with volume values when volume member being an integer.

This method recieves double value between 0.0 and 1.0 and sets m_volume to 0 or 1 (if value being 1.0) : 

bool QTextToSpeechProcessor::setVolume(double volume)
{
    QMutexLocker lock(&m_lock);
    if (volume >= 0.0 && volume <= 1.0) {
        if (updateVolume(volume)) {
            m_volume = volume;
            qDebug() << "m_volume is : " << m_volume;
            return true;
        }
    }
    return false;
}

When member changed to double it works as it should.