Problems with binding to ColumnDefinition Width in grid?

Another hour, another problem..

Anyone else having problems with binding to a ColumnDefinition Width?

This is my xaml:

        <Grid BackgroundColor="#f00" >
            <Grid.RowDefinitions>
                <RowDefinition Height="Auto" />
            </Grid.RowDefinitions>

            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="{Binding HalfScreenWidth}" />
                <ColumnDefinition Width="{Binding HalfScreenWidth}" />
            </Grid.ColumnDefinitions>

            <Image Source="xx.png" Grid.Row="0" Grid.Column="0" BackgroundColor="#0f0" />
            <Image Source="xx.png" Grid.Row="0" Grid.Column="1" BackgroundColor="#00f" />
        </Grid>

in my view model I define HalfScreenWidth like this:

    public GridLength HalfScreenWidth { get; set; }

I have tried with different input, e.g.:

        HalfScreenWidth = new GridLength (50);

But it's all ignored.

Another bug? Or me misunderstanding something?

I can bind to the property from a Label with no problem like this:

<ContentPage.Resources>
    <ResourceDictionary>
        <local:GridLengthToStringConverter x:Key="gridLengthConverter" />
    </ResourceDictionary>
</ContentPage.Resources>

<Label Text="{Binding HalfScreenWidth, Converter={StaticResource gridLengthConverter}"  />

Having implemented a converter like this:

class GridLengthToStringConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        var gl = (GridLength)value;
        return String.Format("Gridlength: {0}", gl.Value);
    }
    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        return new GridLength (10); // not used..
    }
}

The label shows the expected string (e.g. "Gridlength: 50")
Thanks in advance,