Disabled buttons are invisible on WinPhone 8.1 Silverlight

I have the following trigger for styling disabled button:

            <Style TargetType="Button">
                <Setter Property="BackgroundColor" Value="#017A73" />
                <Setter Property="TextColor" Value="#fffefe" />
                <Setter Property="FontFamily" Value="{StaticResource FaktSmConProMedium}" />
                <Setter Property="HeightRequest">
                    <Setter.Value>
                        <OnPlatform Android="45"
                                    WinPhone="90"
                                    iOS="50"
                                    x:TypeArguments="x:Double" />
                    </Setter.Value>
                </Setter>
                <Setter Property="FontSize" Value="{StaticResource ButtonTextSize}" />
                <Style.Triggers>
                  <Trigger Property="IsEnabled" Value="False" TargetType="Button">
                      <Setter Property="TextColor" Value="#909090" />
                      <Setter Property="BackgroundColor" Value="#C0C0C0" />
                  </Trigger>
              </Style.Triggers>
            </Style>

This works on both iOS and Android but on WinPhone 8.1 Silverlight the button simply disappears when it's disabled. It is there as it can be tapped but it's not visibla at all.

Any ideas?