Check Mark Showing as a Black Box in Menu

I have a menu inside a toolbar that needs to be checkable but when I check the items it renders as just a black box (I tried to upload a photo of the issue but StackOverflow wouldn't let me). When you mouse over the check mark renders correctly. I don't understand it I have tried removing any styles that might be affecting it but nothing seems to affect it.

Can anyone help?

Here is the XAML for the menu item:

<ToolBar ToolBarTray.IsLocked="True" Margin="0" Padding="0" ToolBar.OverflowMode="Never">
    <Button Command="com:FormatCommands.LogInCommand">
        <Viewbox Width="15">
            <Image Source="..\Resources\Login.png"/>
        </Viewbox>                        
    </Button>
    <Button Command="com:FormatCommands.LogOutCommand">
        <Viewbox Width="15">
            <Image Source="..\Resources\Logout.png"/>
        </Viewbox>
    </Button>
    <Button Command="com:FormatCommands.GetTodaysRacesCommand">
        <Viewbox Width="15">
            <Image Source="..\Resources\list.png"/>
        </Viewbox>
    </Button>
    <Menu>
        <MenuItem Header="Markets">
            <MenuItem IsCheckable="True" Header="British"/>
            <MenuItem IsCheckable="True" Header="Irish" />
        </MenuItem>
    </Menu>            
</ToolBar>