Radcombobox clears the text when it is not in the items list

I have a telerik's radcombobox like below:

<telerik:RadComboBox x:Name="cbCustomerName"
                                 Grid.Row="1"
                                 Grid.Column="1"
                                 Grid.ColumnSpan="3"
                                 Margin="0 0 0 10"
                                 DisplayMemberPath="FullName"
                                 IsEditable="True"
                                 IsFilteringEnabled="True"
                                 ItemsSource="{Binding Customers}"
                                 OpenDropDownOnFocus="True"
                                 SelectedValuePath="CustomerId"
                                 SelectionChanged="cbCustomerName_SelectionChanged"
                                 Text="{Binding CurrentCustomer.FullName,
                                                UpdateSourceTrigger=LostFocus}" />

When I type something in the textbox that is not in its items list and then go to some other control, it clears the text in it, which I don't want.

I have checked all properties and couldn't find anything to retain it after losing focus.

Please help if anyone knows about this.