Error Rendering Control

Hi,
I'm using VS2005 and i'm currently building a custom web control librairies. For example i created a control derived from a textbox and i added a string "BoundProperty" property.
Here's the code of this control
<DefaultProperty("Text"), ToolboxData("<{0}:CSTextBox runat=server></{0}:CSTextBox>")> _
Public Class CSTextBox
    Inherits WebControls.TextBox
    Implements IBindableCSControl

    Dim m_sBoundProperty As String
    '<Browsable(True), Description("Property name of the associated control")> _
    Public Property BoundProperty() As String Implements IBindableCSControl.BoundProperty
        Get
            Return m_sBoundProperty
    ...