Second button does not work

[programmher]

I have two drop down boxes and one list box. My client wants the patrons to either search for authors from a drop down box that lists all the authors or from the list box that displays a list of authors based on the genre. The individual authors add button does not work. I get an error message that states: Exception has been thrown by the target of an invocation.

Here is my code:

protected void AddAnAuthor(object sender, EventArgs e)
    {
        AddAuthors_ds.Insert();
        OneAuthorList_lb.ClearSelection();
        Response.Redirect(Request.Url.AbsoluteUri);
    }
   protected void AddAuthors(object sender, EventArgs e)
    {
        AddAuthors_ds.Insert();
        AuthorList_lb.ClearSelection();
        Response.Redirect(Request.Url.AbsoluteUri);
    }

<asp:DropDownList runat="server" ID="OneAuthorList_lb" style="float:left;" DataSourceID="AuthorList" DataTextField="AuthorName" DataValueField="AuthorName"  onselectedindexchanged="AddOneAuthor_SelectedIndexChanged" 
                 AutoPostBack="True" AppendDataBoundItems="true" />
<asp:ImageButton ImageUrl="add.png"  OnMouseOver="addHover.png'" runat="server"  ID="AddAnAuthor" style="float:left; margin-left:5px;" OnClick="AddAnAuthor" />

<asp:DropDownList runat="server" ID="AuthorsList" 
                 style="float:left;" DataSourceID="AuthorsList_ds" 
                 DataTextField="AuthorName" DataValueField="AuthorName" 
                 onselectedindexchanged="AuthorsList_SelectedIndexChanged" 
                 AutoPostBack="True" AppendDataBoundItems="true">
    <asp:ListItem Text="Select Genre" Value="~Select Genre~" />         
</asp:DropDownList> 
<asp:ImageButton ImageUrl="add.png'" ToolTip="Add Authors From List" runat="server"  ID="btnAddAuthors" style="float:left; margin-left:5px;" OnClick="btnAddAuthorsListed" />
 
       <asp:ListBox runat="server" ID="AuthorList" DataTextField="AuthorName" 
            DataValueField="UserID" DataSourceID="Authors" SelectionMode="Multiple"  AppendDataBoundItems="true" />

How do I resolve the error and meet my client's requirement to allow library patrons to use either button? Both buttons need to fire the same event.