bugfix: date type casting bug

data/maker/__init__.py

@@ -284,6 +284,7 @@ class Generator (Learner):
                     _df[name] = _df[name].astype('datetime64[ns]')
                 else:
                     _df[name] = _df[name].astype(str)
+                _df[name] = _df[name].fillna('')
         if r :
             self.log(**{'action':'format','input':r})
         return _df