Material style ComboBox has white text (on a white background) when put in a ToolBar

import QtQuick 2.6
import QtQuick.Layouts 1.3
import QtQuick.Controls 2.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    header: ToolBar {
        ComboBox {
            model: Qt.fontFamilies()
        }
    }

    ComboBox {
        model: Qt.fontFamilies()
        anchors.centerIn: parent
    }
}