Qt 5.5.0 for Android QPixmap fill with Qt::transparent is black

Hello, When I port my project from Qt5.4.0 to Qt5.5.0 on Android, I find the QPixmap fill with transparent can not work, always show black, but on Qt5.4.0(Android) works well:
QPixmap *result = new QPixmap(400, 300);
result->fill(Qt::transparent);// display is black, why;

I need your help, when run my project on Android with Qt5.4.0; I also have progame that is when I open another apk from my Qt apk, after I exit the apk(not Qt apk), and on some Android device my Qt apk is black(the Android screen is black). Now I build my Qt apk with Qt5.5.0 is working ok, but the QPixmap fill transparent show black; Now I'm confuse, I need your help