asp:textbox text gets clear when radiobutton has autopostback enabled

Hi ,
I have a radiobutton list on one of my UserControl(code in C#) with AutoPostBack = true
asp:RadioButtonList runat="server" RepeatColumns="7" Width="100%" ID="GalleryCheck" DataTextField="imageid" DataTextFormatString="Gallery-{0}" DataValueField="imageid" RepeatDirection="Horizontal" OnSelectedIndexChanged="GalleryCheck_SelectedIndexChanged" EnableViewState="true" AutoPostBack="true" OnDataBinding="GalleryCheck_DataBinding"></asp:RadioButtonList> Then I am using the above UserControl on another UserControl which has a asp:textbox control. The text value of the asp:textbox control gets clear when I click any of the radiobutton from the rbl.
<asp:TextBox ID="richbox" runat="server" EnableViewState="true" TextMode="MultiLine"></asp:TextBox> What I ...