Grid row height binding does not work

Basically in the XAML below, the binding is ignored and the property Get never called. There are also no binding related messages.

<Grid >
    <Grid.RowDefinitions>
        <RowDefinition Height="*"/>
        <RowDefinition Height="{Binding Path=ToolbarHeight,Mode=TwoWay}" />
    </Grid.RowDefinitions>
    .....
</Grid>

Other VM bindings work and also log debug errors when wrong, but it looks like something is not implemented for grid row height?

The VM bits are as follows and appear correct so the problem appears to be on the Xamarin side of things.

    private GridLength toolbarHeight;
    public GridLength ToolbarHeight
    {
        get
        {
            return toolbarHeight;  // Never gets called
        }
        set
        {
            SetProperty(ref toolbarHeight, value);
        }
    }

This is with the latest alpha bits as of today. I don't know if this was an issue with previous builds because I just started using Xamarin.Forms.

Cheers