Newbie Help! SqlDataSource + Select Parameter

 Why does this not work?

        <asp:SqlDataSource ID="employeeSource"
            runat="server"
            ConnectionString="<%$ ConnectionStrings:mainDB %>"
            DataSourceMode="DataReader"
            ProviderName="System.Data.SqlClient"
            OnSelecting="OnSourceSelecting"
            SelectCommand="SELECT * FROM Employees WHERE ID = @employeeID">
            
            <SelectParameters>
                <asp:Parameter Name="@employeeID" Type="Int32" />
            </SelectParameters>
        </asp:SqlDataSource>

In code behind:

        protected void OnSourceSelecting(object sender, SqlDataSourceCommandEventArgs args) {
            // THIS LINE THROWS EXCEPTION: An SqlParameter with ParameterName '@campaignID' is not contained by this SqlParameterCollection.
            args.Command.Parameters["@employeeID"].Value = 3;    // In future, will use dynamic value.
        }
        
If, instead I do this:    
        
        <asp:SqlDataSource ID="employeeSource"
            runat="server"
            ConnectionString="<%$ ConnectionStrings:mainDB %>"
            DataSourceMode="DataReader"
            ProviderName="System.Data.SqlClient"
            OnSelecting="OnSourceSelecting"
            SelectCommand="SELECT * FROM Employees WHERE ID = @employeeID">
            
            <SelectParameters>
                <asp:QueryStringParameter Name="@campaignID" QueryStringField="id" />
            </SelectParameters>
        </asp:SqlDataSource>

(Of course I call page with EmployeePage.aspx?id=123)
I get the exception: Must declare the scalar variable "@employeeID".