diff --git a/test/test_config.py b/test/test_config.py
index d66e27e..cacb7d6 100644
--- a/test/test_config.py
+++ b/test/test_config.py
@@ -264,6 +264,23 @@ def test_config_multi():
     assert config.option('test3').value.get() == [2, 1]
 
 
+def test_prefix_error():
+    i1 = IntOption('test1', '')
+    od = OptionDescription('test', '', [i1])
+    config = Config(od)
+    config.property.read_write()
+    config.option('test1').value.set(1)
+    try:
+        config.option('test1').value.set('yes')
+    except Exception as err:
+        assert str(err) == '"yes" is an invalid integer for "test1"'
+    try:
+        config.option('test1').value.set('yes')
+    except Exception as err:
+        err.prefix = ''
+        assert str(err) == 'invalid value'
+
+
 def test_no_validation():
     i1 = IntOption('test1', '')
     od = OptionDescription('test', '', [i1])
diff --git a/test/test_mandatory.py b/test/test_mandatory.py
index 0afe902..c3966c8 100644
--- a/test/test_mandatory.py
+++ b/test/test_mandatory.py
@@ -417,6 +417,7 @@ def test_mandatory_leader():
     api = Config(descr)
     api.property.read_only()
     raises(PropertiesOptionError, "api.option('ip_admin_eth0.ip_admin_eth0').value.get()")
+    raises(PropertiesOptionError, "api.value.dict()")
 
 
 def test_mandatory_warnings_leader():
diff --git a/test/test_metaconfig.py b/test/test_metaconfig.py
index cfb24aa..4a1ae52 100644
--- a/test/test_metaconfig.py
+++ b/test/test_metaconfig.py
@@ -63,6 +63,12 @@ def test_unknown_config():
     raises(ConfigError, "meta.config('unknown')")
 
 
+def test_error_metaconfig():
+    od2 = make_description()
+    conf1 = Config(od2, session_id='conf1')
+    raises(TypeError, "MetaConfig([GroupConfig([conf1])], session_id='meta')")
+
+
 def test_path():
     meta = make_metaconfig()
     assert meta.config.path() == 'meta'
diff --git a/test/test_option_consistency.py b/test/test_option_consistency.py
index 137ec99..9097997 100644
--- a/test/test_option_consistency.py
+++ b/test/test_option_consistency.py
@@ -88,6 +88,24 @@ def test_consistency_warnings_only_more_option():
     assert len(w) == 1
 
 
+def test_consistency_error_prefix():
+    a = IntOption('a', '')
+    b = IntOption('b', '')
+    od = OptionDescription('od', '', [a, b])
+    a.impl_add_consistency('not_equal', b)
+    api = Config(od)
+    api.option('a').value.set(1)
+    try:
+        api.option('b').value.set(1)
+    except Exception as err:
+        assert str(err) == '"1" is an invalid integer for "b", must be different from the value of "a"'
+    try:
+        api.option('b').value.set(1)
+    except Exception as err:
+        err.prefix = ''
+        assert str(err) == 'must be different from the value of "a"'
+
+
 def test_consistency_warnings_only_option():
     a = IntOption('a', '')
     b = IntOption('b', '', warnings_only=True)
diff --git a/tiramisu/config.py b/tiramisu/config.py
index b0cbb3d..4ecfc6f 100644
--- a/tiramisu/config.py
+++ b/tiramisu/config.py
@@ -1198,12 +1198,13 @@ class KernelMetaConfig(KernelMixConfig):
             if not isinstance(child, _CommonConfig):
                 try:
                     child = child._config
-                except:
+                except Exception:
                     raise TypeError(_("{}config's children "
                                       "should be config, not {}"
                                       ).format(self.impl_type,
                                                type(child)))
-            if not isinstance(child, (KernelConfig, KernelMetaConfig)):
+            if __debug__ and not isinstance(child, (KernelConfig,
+                                                    KernelMetaConfig)):
                 raise TypeError(_("child must be a Config or MetaConfig"))
             if descr is None:
                 descr = child.cfgimpl_get_description()
diff --git a/tiramisu/option/netmaskoption.py b/tiramisu/option/netmaskoption.py
index cc7af39..5fbfda5 100644
--- a/tiramisu/option/netmaskoption.py
+++ b/tiramisu/option/netmaskoption.py
@@ -19,6 +19,7 @@
 # the whole pypy projet is under MIT licence
 # ____________________________________________________________
 from ipaddress import ip_interface, ip_network
+from typing import List
 
 from ..error import ConfigError
 from ..setting import undefined, OptionBag, Undefined
@@ -49,11 +50,11 @@ class NetmaskOption(StrOption):
             raise ValueError()
 
     def _cons_network_netmask(self,
-                              current_opt,
-                              opts,
-                              vals,
-                              warnings_only,
-                              context):
+                              current_opt: Option,
+                              opts: List[Option],
+                              vals: List[str],
+                              warnings_only: bool,
+                              context: 'Config'):
         if context is undefined and len(vals) != 2:
             raise ConfigError(_('network_netmask needs a network and a netmask'))
         if None in vals or len(vals) != 2:
@@ -71,12 +72,12 @@ class NetmaskOption(StrOption):
                                                                         opt_network.impl_get_display_name()))
 
     def _cons_ip_netmask(self,
-                         current_opt,
-                         opts,
-                         vals,
-                         warnings_only,
-                         context,
-                         _cidr=False):
+                         current_opt: Option,
+                         opts: List[Option],
+                         vals: List[str],
+                         warnings_only: bool,
+                         context: 'config',
+                         _cidr: bool=False):
         if context is undefined and len(vals) != 2:
             raise ConfigError(_('ip_netmask needs an IP and a netmask'))
         if None in vals or len(vals) != 2: