I'm attempting to position my perspective camera 30 units above the origin and pointing straight down.

If I set the LookDirection of the camera to "0,0,-1", however, everything disappears. I have to make it "0.01,0.01,-1" for it to work. Why?

<Window
    x:Class="ThreeDeeTester.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Window1"
    Height="300"
    Width="300">
    <Grid>
        <Viewport3D>
            <Viewport3D.Camera>
                <PerspectiveCamera
                    Position="0,0,30"
                    LookDirection="0.01,0.01,-1" 
                    UpDirection="0,0,1" />
                    <!-- LookDirection="0,0,-1" doesn't work...why? -->
            </Viewport3D.Camera>
            <ModelVisual3D>
                <ModelVisual3D.Content>
                    <Model3DGroup>
                        <DirectionalLight
                            Color="White"
                            Direction="1,-1,-1" />
                        <GeometryModel3D>
                            <GeometryModel3D.Geometry>
                                <MeshGeometry3D
                                    Positions="0,0,10 -5,-5,0 -5,5,0 5,5,0 5,-5,0"
                                    TriangleIndices="2 1 0  2 0 3  4 3 0  1 4 0" />
                            </GeometryModel3D.Geometry>
                            <GeometryModel3D.Material>
                                <DiffuseMaterial Brush="Red" />
                            </GeometryModel3D.Material>
                        </GeometryModel3D>
                    </Model3DGroup>
                </ModelVisual3D.Content>
            </ModelVisual3D>
        </Viewport3D>
    </Grid>
</Window>