Wrongly painted background of QPushButton and QToolButton

the stylesheet

QPushButton, QToolButton

{ background-color: qlineargradient(x1: 0, y1: 0, x2: 0, y2: 1, stop: 0 #5c8dc2, stop: 1 #9ec2e9); }

makes all the button filled with completely black background.

this happens when qlineargradient is applied as background.

However the same stylesheet having additional items like:

QPushButton, QToolButton

{ background-color: qlineargradient(x1: 0, y1: 0, x2: 0, y2: 1, stop: 0 #5c8dc2, stop: 1 #9ec2e9); color: white; }

will be painted correctly.