diff --git a/test/test_metaconfig.py b/test/test_metaconfig.py
index c461b25..a013caa 100644
--- a/test/test_metaconfig.py
+++ b/test/test_metaconfig.py
@@ -1,8 +1,7 @@
+from py.test import raises
 from .autopath import do_autopath
 do_autopath()
 
-from py.test import raises
-
 from tiramisu.setting import groups, owners
 from tiramisu import IntOption, StrOption, NetworkOption, NetmaskOption, \
                      OptionDescription, MasterSlaves, Config, GroupConfig, MetaConfig, \
diff --git a/test/test_mixconfig.py b/test/test_mixconfig.py
index 408a7f4..f1dcaaa 100644
--- a/test/test_mixconfig.py
+++ b/test/test_mixconfig.py
@@ -90,6 +90,15 @@ def make_mixconfig(double=False):
     return mix
 
 
+def test_mix_not_group():
+    i1 = IntOption('i1', '')
+    od1 = OptionDescription('od1', '', [i1])
+    od2 = OptionDescription('od2', '', [od1])
+    conf1 = Config(od2, session_id='conf1')
+    grp = GroupConfig([conf1])
+    raises(TypeError, "MixConfig(od2, [grp])")
+
+
 def test_unknown_config():
     mix = make_mixconfig()
     raises(ConfigError, "mix.config('unknown')")
@@ -271,63 +280,6 @@ def test_mix_mix_set():
     raises(ValueError, "mix.value.set('od1.i6', 7, only_config=True, force_dont_change_value=True)")
 
 
-def test_not_mix():
-    i1 = IntOption('i1', '')
-    od1 = OptionDescription('od1', '', [i1])
-    od2 = OptionDescription('od2', '', [od1])
-    conf1 = Config(od2, session_id='conf1')
-    conf2 = Config(od2, session_id='conf2')
-    conf3 = Config(od2)
-    conf4 = Config(od2, session_id='conf4')
-    raises(TypeError, "GroupConfig(conf1)")
-    #same name
-    #raises(ConflictError, "GroupConfig([conf2, conf4], session_id='conf2')")
-    raises(ConflictError, "GroupConfig([conf2, conf2], session_id='conf8')")
-    grp = GroupConfig([conf1, conf2])
-    raises(APIError, "grp.option('od1.i1').value.get()")
-    conf1, conf2 = grp.config.list()
-    errors = grp.value.set('od1.i1', 7)
-    assert len(errors) == 0
-    assert grp.config('conf1').option('od1.i1').value.get() == grp.config('conf2').option('od1.i1').value.get() == 7
-    assert grp.config('conf1').option('od1.i1').owner.get() is grp.config('conf2').option('od1.i1').owner.get() is owners.user
-    grp.option('od1.i1').value.reset()
-    assert grp.config('conf1').option('od1.i1').owner.get() is grp.config('conf2').option('od1.i1').owner.get() is owners.default
-
-
-def test_group_find_firsts():
-    i1 = IntOption('i1', '')
-    od1 = OptionDescription('od1', '', [i1])
-    od2 = OptionDescription('od2', '', [od1])
-    conf1 = Config(od2, session_id='conf1')
-    conf2 = Config(od2, session_id='conf2')
-    grp = GroupConfig([conf1, conf2])
-    itr = grp.config.find('i1').config.list()
-    conf1._config_bag.context == next(itr)._config_bag.context
-    conf2._config_bag.context == next(itr)._config_bag.context
-    try:
-        next(itr)
-    except StopIteration:
-        pass
-    except:
-        raise Exception('no')
-    else:
-        raise Exception('no')
-
-
-def test_group_group():
-    i1 = IntOption('i1', '')
-    od1 = OptionDescription('od1', '', [i1])
-    od2 = OptionDescription('od2', '', [od1])
-    conf1 = Config(od2, session_id='conf9')
-    conf2 = Config(od2, session_id='conf10')
-    grp = GroupConfig([conf1, conf2], 'grp')
-    grp2 = GroupConfig([grp])
-    errors = grp2.value.set('od1.i1', 2)
-    assert len(errors) == 0
-    assert grp2.config('grp.conf9').option('od1.i1').value.get() == 2
-    assert grp2.config('grp.conf9').option('od1.i1').owner.get() is owners.user
-
-
 def test_mix_unconsistent():
     i1 = IntOption('i1', '')
     i2 = IntOption('i2', '', default=1)
diff --git a/tiramisu/config.py b/tiramisu/config.py
index 30cf507..a0ab83f 100644
--- a/tiramisu/config.py
+++ b/tiramisu/config.py
@@ -980,6 +980,8 @@ class KernelMixConfig(KernelGroupConfig):
                                       "should be config, not {}"
                                       ).format(self.impl_type,
                                                type(child)))
+            if not isinstance(child, (KernelConfig, KernelMixConfig)):
+                raise TypeError(_("child must be a Config, MixConfig or MetaConfig"))
             if child.cfgimpl_get_meta() is not None:
                 raise ValueError(_("child has already a {}config's").format(self.impl_type))
             child._impl_meta = weakref.ref(self)
@@ -1193,6 +1195,8 @@ class KernelMetaConfig(KernelMixConfig):
                                       "should be config, not {}"
                                       ).format(self.impl_type,
                                                type(child)))
+            if not isinstance(child, (KernelConfig, KernelMetaConfig)):
+                raise TypeError(_("child must be a Config or MetaConfig"))
             if descr is None:
                 descr = child.cfgimpl_get_description()
             elif descr is not child.cfgimpl_get_description():