diff --git a/tiramisu/error.py b/tiramisu/error.py
index 55732b1..d6624f5 100644
--- a/tiramisu/error.py
+++ b/tiramisu/error.py
@@ -29,11 +29,20 @@
 #AttributeError if no option or optiondescription in optiondescription (also when specified a path)
 
 
+#____________option___________
+class PropertiesOptionError(AttributeError):
+    "try to access to opt with not allowed property"
+    def __init__(self, msg, proptype):
+        self.proptype = proptype
+        super(PropertiesOptionError, self).__init__(msg)
+
+
 class ConflictOptionError(StandardError):
     "more than one option"
     pass
 
 
+#____________config___________
 class ConfigError(StandardError):
     """try to change owner for an option without value
     or if error in calculation"""
@@ -45,13 +54,12 @@ class ConflictConfigError(ConfigError):
     pass
 
 
-class PropertiesOptionError(AttributeError):
-    "try to access to opt with not allowed property"
-    def __init__(self, msg, proptype):
-        self.proptype = proptype
-        super(PropertiesOptionError, self).__init__(msg)
-
-
+#____________other___________
 class RequirementRecursionError(StandardError):
     "recursive error"
     pass
+
+
+class SlaveError(StandardError):
+    "problem with slave's length"
+    pass
diff --git a/tiramisu/value.py b/tiramisu/value.py
index 08a6f55..5838df6 100644
--- a/tiramisu/value.py
+++ b/tiramisu/value.py
@@ -18,7 +18,7 @@
 #
 # ____________________________________________________________
 from time import time
-from tiramisu.error import ConfigError
+from tiramisu.error import ConfigError, SlaveError
 from tiramisu.setting import owners, multitypes, expires_time
 from tiramisu.autolib import carry_out_calculation
 from tiramisu.i18n import _
@@ -214,7 +214,7 @@ class Multi(list):
         masterlen = len(mastervalue)
         if len(value) > masterlen or (len(value) < masterlen and
                                       not self.context.cfgimpl_get_values().is_default_owner(self.opt)):
-            raise ValueError(_("invalid len for the slave: {0}"
+            raise SlaveError(_("invalid len for the slave: {0}"
                                " which has {1} as master").format(
                                    self.opt._name, masterp))
         elif len(value) < masterlen:
@@ -230,7 +230,7 @@ class Multi(list):
             if not values.is_default_owner(slave):
                 value_slave = values._get_value(slave)
                 if len(value_slave) > masterlen:
-                    raise ValueError(_("invalid len for the master: {0}"
+                    raise SlaveError(_("invalid len for the master: {0}"
                                        " which has {1} as slave with"
                                        " greater len").format(
                                            self.opt._name, slave._name))
@@ -250,7 +250,7 @@ class Multi(list):
         """
         if not force:
             if self.opt.get_multitype() == multitypes.slave:
-                raise ValueError(_("cannot append a value on a multi option {0}"
+                raise SlaveError(_("cannot append a value on a multi option {0}"
                                    " which is a slave").format(self.opt._name))
             elif self.opt.get_multitype() == multitypes.master:
                 for slave in self.opt.get_master_slaves():
@@ -278,7 +278,7 @@ class Multi(list):
         """
         if not force:
             if self.opt.get_multitype() == multitypes.slave:
-                raise ValueError(_("cannot pop a value on a multi option {0}"
+                raise SlaveError(_("cannot pop a value on a multi option {0}"
                                    " which is a slave").format(self.opt._name))
             elif self.opt.get_multitype() == multitypes.master:
                 for slave in self.opt.get_master_slaves():