Error message loops indefinitelly when more than 255 characters

We use the MultiValuControl to indicate customers related to a specicif WI. When the length of the selected customer names reach 256 we get a message indicating "The set of values selected by the multiple value control in Customer Name exceeds the 255 character limit" witch is right but the problem is we can't get out of it. Each time we click on OK the same message reappears.   The only way out is to kill Visual Studio...