Required field validator for User control not working

Hi I have created user control for re-sizable text box.

<asp:Panel ID="PanelText" runat="server" CssClass="frameText">
 <asp:TextBox runat="server" ID="TextBoxResizable" CssClass="noborder" Width="100%"
     Height="100%" TextMode="MultiLine">
 </asp:TextBox>
 </asp:Panel>
 <cc1:ResizableControlExtender ID="ResizableTextBoxExtender" runat="server" TargetControlID="PanelText"
 ResizableCssClass="resizingText" HandleCssClass="handleText" OnClientResizing="OnClientResizeText" />
 

And have created Validator property for this control like:

[ValidationProperty("Text")]
  public partial class ResizableTextBoxControl : System.Web.UI.UserControl
 { public string Validator
 {
     get { return this.TextBoxResizable.Text; }
 } 
 protected void Page_Load(object sender, EventArgs e)
  {
 
  }
 }
 

In aspx page I am using this control like:

<uc1:ResizableTextBoxControl ID="tbDescription" MinimumHeight="50" MinimumWidth="100"
  MaximumHeight="300" MaximumWidth="400" runat="server" onKeyPress="javascript:Count(this,1500);" onKeyUp="javascript:Count(this,1500);" ValidationGroup="vgSubmit" ></uc1:ResizableTextBoxControl>
 
  <asp:RequiredFieldValidator ID="rfvDescription" runat="server" controlToValidate="tbDescription" ValidationGroup="vgSubmit" ErrorMessage="Description" Text="*" ForeColor="Red" SetFocusOnError="True"></asp:RequiredFieldValidator>
 

When I click on submit, "tbDescription" does not appear to be mandatory.
What can be wrong with the code?

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