WPF NumericUpDown usercontrol in VB.net

I am creating my own NumericUpDown control in VB. Here is my XAML:

<UserControl xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             x:Class="NumericUpDown"
             mc:Ignorable="d" 
             d:DesignHeight="30" d:DesignWidth="100">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="20" />
            <ColumnDefinition Width="20" />
        </Grid.ColumnDefinitions>
        <TextBox x:Name="txtNum" Grid.Column="0" x:FieldModifier="private" TextChanged="txtNum_TextChanged"/>
        <Button x:Name="cmdDown" Grid.Column="1" x:FieldModifier="private" Content="˅" Width="20" Click="cmdDown_Click" />
        <Button x:Name="cmdUp" Grid.Column="2" x:FieldModifier="private" Content="˄" Width="20" Click="cmdUp_Click" />
    </Grid>
</UserControl>

And here is the VB code behind it:

Class NumericUpDown

    Dim _Minimum As Double = 0
    Dim _Maximum As Double = 100

    Private Sub NumericUpDown()
        InitializeComponent()
        txtNum.Text = Numeric
    End Sub

    Public Property Maximum As Double
        Get
            Return _Maximum
        End Get
        Set(value As Double)
            _Maximum = value
        End Set
    End Property

    Public Property Minimum As Double
        Get
            Return _Minimum
        End Get
        Set(value As Double)
            _Minimum = value
        End Set
    End Property

    Public Shared ReadOnly NumericProperty As DependencyProperty = DependencyProperty.Register("Numeric", GetType(String), GetType(NumericUpDown), _
        New PropertyMetadata(""))

    Public Property Numeric As String
        Get
            Return CType(GetValue(NumericProperty), String)
        End Get
        Set(value As String)
            SetValue(NumericProperty, value)
        End Set
    End Property

    Private Sub cmdUp_Click(sender As Object, e As RoutedEventArgs)
        Dim NumValue As Double
        NumValue = Val(txtNum.Text)
        NumValue += 1
        If NumValue > Maximum Then NumValue = Maximum
        txtNum.Text = NumValue.ToString
    End Sub

    Private Sub cmdDown_Click(sender As Object, e As RoutedEventArgs)
        Dim NumValue As Double
        NumValue = Val(txtNum.Text)
        NumValue -= 1
        If NumValue < Minimum Then NumValue = Minimum
        txtNum.Text = NumValue.ToString
    End Sub

    Private Sub txtNum_TextChanged(sender As Object, e As TextChangedEventArgs)
        Numeric = txtNum.Text
    End Sub
End Class

I use it in my page like this: Put this on page definition:

xmlns:local="clr-namespace:Demo"

And put this in the content section:

<local:NumericUpDown Numeric="{Binding Path=score, Mode=TwoWay, NotifyOnValidationError=true, ValidatesOnExceptions=true}"/>

Of course I have already set DataContext on the container and all other databound controls work as they should. But the textbox in my custom control turned out empty! It doesn't end here. When I type something in the textbox and when I give it some value using decrease and increase button, the value is transferred to my DataTable; which means this usercontrol does work to some extend. Where did I do wrong? Why won't the Textbox content be initialized with the starting value?

After a little more testing. It seems that my usercontrol doesn't work in 'Two-Way'. It doesn't receive data from DataTable, it only propagates value to it. How do I fix it?