How to modify sql Data Source parameters on event?

here is my scenario:

Iv linked a parameter of my sqlds to a dropDownList (dropDownList.selectedValue), but i want it so that when the selected item is "" (an item with no text is added on pageload, to let the user select a null value, sort of), i want the sqlds to pass null. Here is what i tried, but it doesnt开发者_StackOverflow中文版 work (gave the error saying no such parameter existed:

protected void sqldsNewItem_Inserting(object sender, SqlDataSourceCommandEventArgs e)
{
    DropDownList ddl = dwNewItem.FindControl("ddlEffects") as DropDownList;
    if (ddl.SelectedItem.Text == "")
    {
        e.Command.Parameters["effect"].Value = null;
        e.Command.Parameters["numberOfTurns"].Value = null;
    }
}

(I used FindControl because the dropDownList in question is in a template.)

And here is how i bound it to the sqlds in the first place:

                    <asp:ControlParameter ControlID="dwNewItem$ddlEffects" Name="effect" PropertyName="SelectedValue"
                    DbType="int64" />

Thanks guys!