Combobox causes postback when item is selected by pressing Enter key.

I use two Ajax Control Toolkit comboboxes in my website, both of them are bound dynamically. When I try to select an item using enter key a postback occurs. Please help me with this.

                    <asp:ComboBox ID="ComboBoxCustName" runat="server" DataTextField="Customer" 
                        DataValueField="UserLoginID" 
                        AutoCompleteMode="SuggestAppend" DropDownStyle="Simple">
                    </asp:ComboBox>