Assigning Combobox SelectedItem.Text a String
===

I am receiving the following exception when trying to assign the value from 'model' into a Combobox named ModelNumTB.

For some reason I cannot get it to copy over the value of the string into the default loading value of the Combobox.  How are you supposed to do this?
     string model;

     <ajaxToolkit:ComboBox ID="ModelNumTB" runat="server" 
                        AutoCompleteMode="Suggest" AutoPostBack="True" DataSourceID="ComboboxDS" 
                        DataTextField="MODEL_NUMBER" DataValueField="MODEL_NUMBER" MaxLength="0" 
                        style="display: inline;" 
                        onselectedindexchanged="ModelNumTB_SelectedIndexChanged"></ajaxToolkit:ComboBox>

![Null Reference Exception][1]


  [1]: http://i.stack.imgur.com/WIf35.png