[Zope] Ivalid Integer from Zope

I am using the Znolk Input Wizzard with postgresql tables. One of the variables in the table is an integer and can have null values. However, in the update and insert forms, if I attempt to leave the value null, I get the following error from Zope: "Invalid integer value for weight.bcs" The variable is defined throughout as :<dtml-var bcs null=""> Any help would be appreciated.