Bound CommandParameter on MenuItem is null - wpf

Question

I don't understand why my commandparameter is null when all of the rest of the bindings seem to work, the correct image is displayed, the text is correct and the command CanExecute is called, but for some reason the parameter is null.
<MenuItem Header="Open Recent" ItemsSource="{Binding Path=MRU}">
<MenuItem.ItemContainerStyle>
<Style TargetType="MenuItem">
<Setter Property="Command" Value="{Binding Command}" />
<Setter Property="CommandParameter" Value="{Binding URI}" />
<Setter Property="Header" Value="{Binding URI}" />
<Setter Property="Icon">
<Setter.Value>
<Image Source="{Binding URIImage}" />
</Setter.Value>
</Setter>
</Style>
</MenuItem.ItemContainerStyle>
</MenuItem>
and the view model class:
public class MRU_ViewModel
{
public MRU_ViewModel(string uri)
{
this.URI = uri;
Command = new Commands.Open();
}
public string URI { get; private set; }
public System.Windows.Media.Imaging.BitmapImage URIImage { get { return WorkSpace.GetURIImage(URI); } }
public ICommand Command { get; private set; }
}
and the CanExecute of the command looks like this:
public bool CanExecute(object parameter)
{
return parameter != null && parameter is string; // <---parameter is null
}