Problem databinding a custom control in silverlight

I'm attempting to databind a custom control in Silverlight 3 and I'm getting strange problems with it.

My xaml for the user control is this:

<UserControl x:Class="StronicoMain.GenericSmallIcon"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300" >
    <Canvas x:Name="canGSI">

    </Canvas>
</UserControl>

The codebehind for the user control is this

private string _EntityTypeID;

    public string EntityTypeID
    {
        get
        {
            return _EntityTypeID;
        }

        set
        {
            _EntityTypeID = value;
        }
    }

    public GenericSmallIcon()
    {
        InitializeComponent();
        Loaded += new RoutedEventHandler(Page_Loaded);
    }        
    public void Page_Loaded(object sender, RoutedEventArgs e)
    {
        icoMale icoMale = new icoMale();
        icoFemale icoFem = new icoFemale();

        if (EntityTypeID == null)
        {
            canGSI.Children.Add(icoMale);
        }
        else if (EntityTypeID == "1")
        {
            canGSI.Children.Add(icoMale);
        }
        else if (EntityTypeID == "2")
        {
            canGSI.Children.Add(icoFem);
        }
        else
        {
            canGSI.Children.Add(icoMale);
        }            
    }

I'm calling it from the DataGridSelection Adapter (taken from the Microsoft Toolkit example page for AutoCompleteBox-Datagrid version) - the relevant portion looks like this:

<Stron:DataGridSelectionAdapter x:Name="SelectionAdapter" AutoGenerateColumns="False" IsReadOnly="False">
                                                    <Stron:DataGridSelectionAdapter.Columns>
                                                        <data:DataGridTemplateColumn>
                                                            <data:DataGridTemplateColumn.CellTemplate>
                                                                <DataTemplate><Stron:GenericSmallIcon EntityTypeID="{Binding EntityTypeID}"></Stron:GenericSmallIcon></DataTemplate>
                                                            </data:DataGridTemplateColumn.CellTemplate>
                                                        </data:DataGridTemplateColumn>
                                                            <data:DataGridTextColumn Header="Contact Name" FontWeight="Bold" Foreground="#CC000000" Binding="{Binding EntityName}" />
                                                        <data:DataGridTextColumn Header="Tags" Binding="{Binding EntityTags}" />                                                        
                                                    </Stron:DataGridSelectionAdapter.Columns>
                                                </Stron:DataGridSelectionAdapter>

I run the code, and I get the error "ManagedRuntimeError #4004" - if I try to use the custom control while manually setting the databinding it works just fine, if I try to rely on the values that are being databound I get the error. How can I create a custom databound event on a custom control? I think that is the problem, that the page is loading before the values are passed to it.

Thanks everyone.

~Steve

Update, here is the working, changed code as per the accepted answer

public static readonly DependencyProperty EntityTypeIDProperty = DependencyProperty.Register("EntityTypeID", typeof(string), typeof(GenericSmallIcon), new PropertyMetadata(new PropertyChangedCallback(GenericSmallIcon.OnEntityTypeIDPropertyChanged)));
public string EntityTypeID
{
    get { return (string)GetValue(EntityTypeIDProperty); }
    set { SetValue(EntityTypeIDProperty, value); }
}
private static void OnEntityTypeIDPropertyChanged(
    DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    GenericSmallIcon control = d as GenericSmallIcon;
    string b = (string)e.NewValue;
}