[Zope] Single html checkbox as list?

Hi, since I didn't find anything on that topic, the solution might be obvious but I ran into the following problem: I have a html form with several checkboxes. If one checkbox is selected, the variable containing the result is represented by a string, if two or more checkboxes are selected the variable is represented by a list. Is there a way to force the variable to be a list, even if only one checkbox is selected? greetings clamor