diff --git a/autolib.py b/autolib.py
index c620344..febc172 100644
--- a/autolib.py
+++ b/autolib.py
@@ -19,8 +19,7 @@
 # the whole pypy projet is under MIT licence
 # ____________________________________________________________
 "enables us to carry out a calculation and return an option's value"
-from tiramisu.error import DisabledOptionError, SpecialOwnersError, ConfigError
-from creole import eosfunc
+from tiramisu.error import DisabledOptionError, SpecialOwnersError
 # ____________________________________________________________
 # automatic Option object
 special_owners = ['auto', 'fill']
@@ -52,12 +51,13 @@ def calc_factory(name, callback, callback_params, config):
                 raise DisabledOptionError(e)
             is_multi = opt.is_multi()
             if is_multi:
-                len_value = len(opt_value)
-                if len_multi != 0 and len_multi != len_value:
-                    raise ConfigError('unable to carry out a calculation, '
-                    'option values with multi types must have same length for: '
-                     + name)
-                len_multi = len_value
+                if opt_value != None:
+                    len_value = len(opt_value)
+                    if len_multi != 0 and len_multi != len_value:
+                        raise SpecialOwnersError('unable to carry out a calculation, '
+                        'option values with multi types must have same length for: '
+                         + name)
+                    len_multi = len_value
                 one_is_multi = True
             tcparams[key] = (opt_value, is_multi)
         else:
@@ -69,19 +69,26 @@ def calc_factory(name, callback, callback_params, config):
             tcp = {}
             for key, couple in tcparams.items():
                 value, ismulti = couple
-                if ismulti:
+                if ismulti and value != None:
                     tcp[key] = value[incr]
                 else:
                     tcp[key] = value
-            ret.append(calculate(callback, tcp))
+            ret.append(calculate(name, callback, tcp))
         return ret
     else:
-        return calculate(callback, tcparams)
+        tcp = {}
+        for key, couple in tcparams.items():
+            tcp[key] = couple[0]
+        return calculate(name, callback, tcp)
     
-def calculate(callback, tcparams):
+def calculate(name, callback, tcparams):
     try:
+        # XXX not only creole...
+        from creole import eosfunc
         return getattr(eosfunc, callback)(**tcparams) 
-    except AttributeError:
-        raise SpecialOwnersError("callback: {0} not found for "
-                                           "option: {1}".format(callback, name))
+    except AttributeError, err:
+        import traceback
+        traceback.print_exc()
+        raise SpecialOwnersError("callback: {0} return error {1} for "
+                       "option: {2}".format(callback, str(err), name))
 
diff --git a/config.py b/config.py
index df570d2..b821993 100644
--- a/config.py
+++ b/config.py
@@ -203,9 +203,8 @@ class Config(object):
                         if owner == 'fill' and None not in value:
                             return value
                     else:
-                        if owner == 'fill':
+                        if owner == 'fill' and value != None:
                             return value
-                # ______________________________________________________________
                 result = special_owner_factory(name, owner, 
                             value=value,
                             callback=opt_or_descr.getcallback(),
@@ -233,7 +232,6 @@ class Config(object):
                 else:
                     _result = result
                 return _result
-                # ______________________________________________________________
         # mandatory options
         if not isinstance(opt_or_descr, OptionDescription):
             homeconfig = self._cfgimpl_get_toplevel()
diff --git a/option.py b/option.py
index 3848850..0a3b9d1 100644
--- a/option.py
+++ b/option.py
@@ -34,14 +34,20 @@ group_types = ['default', 'family', 'group', 'master']
 class Option(HiddenBaseType, DisabledBaseType, ModeBaseType):
     #reminder: an Option object is **not** a container for the value
     _frozen = False
-    def __init__(self, name, doc, default=None, requires=None, 
-                 mandatory=False, multi=False, callback=None, 
+    def __init__(self, name, doc, default=None, default_multi=None, 
+                 requires=None, mandatory=False, multi=False, callback=None, 
                  callback_params=None, mode='normal'):
         self._name = name
         self.doc = doc
         self._requires = requires
         self._mandatory = mandatory
         self.multi = multi
+        if not self.multi and default_multi is not None:
+            raise ConfigError("a default_multi is set whereas multi is False"
+                  " in option: {0}".format(name)) 
+        self.default_multi = default_multi
+        #if self.multi and default_multi is None:
+            # _cfgimpl_warnings[name] = DefaultMultiWarning   
         self.callback = callback
         if self.callback is None and callback_params is not None:
             raise ConfigError("params defined for a callback function but"