diff --git a/eo_gestion/eo_facture/fields.py b/eo_gestion/eo_facture/fields.py
index 437f656..cb2b447 100644
--- a/eo_gestion/eo_facture/fields.py
+++ b/eo_gestion/eo_facture/fields.py
@@ -97,6 +97,20 @@ class PercentagePerYearFormField(forms.Field):
         return parse_percentage_per_year(value)
 
 
+class PercentagePerYearDescriptor:
+    def __init__(self, field):
+        self.field = field
+
+    def __set__(self, obj, value):
+        if isinstance(value, str):
+            value = parse_percentage_per_year(value)
+        if isinstance(value, list) and not isinstance(value, PercentagePerYear):
+            value = PercentagePerYear(value)
+        if not isinstance(value, PercentagePerYear):
+            raise ValueError('PercentagePerYear value expected')
+        obj.__dict__[self.field.name] = value
+
+
 class PercentagePerYearField(models.Field):
     default_validators = [check_percentage_per_year]
 
@@ -143,3 +157,7 @@ class PercentagePerYearField(models.Field):
         defaults = {"form_class": PercentagePerYearFormField}
         defaults.update(kwargs)
         return super().formfield(**kwargs)
+
+    def contribute_to_class(self, cls, name, **kwargs):
+        super().contribute_to_class(cls, name, **kwargs)
+        setattr(cls, self.name, PercentagePerYearDescriptor(self))