Defining string as key in dictionary for DB2 database

I am converting from a Websphere environment to SAP NW. My DB2 database has strings in the keys, but when I define them in the NW dictionary I get "Dictionary Generation: SAPDBcheckWidth TABLENAME: index on BLOB, CLOB, LONGVARBINARY or LONGVARCHAR is not allowed". Anybody got any ideas? I have defined my database vendor as DB2_UDB in persistence.xml (in the EJB project), but I can't find anything in the dictionary project to tell it strings are OK!