diff --git a/eo_gestion/eo_facture/fields.py b/eo_gestion/eo_facture/fields.py
index 034864c..437f656 100644
--- a/eo_gestion/eo_facture/fields.py
+++ b/eo_gestion/eo_facture/fields.py
@@ -128,11 +128,13 @@ class PercentagePerYearField(models.Field):
         return value
 
     def get_prep_value(self, value):
-        if isinstance(value, PercentagePerYear):
-            return text_type(value)
-        elif value is not None:
-            return text_type(parse_percentage_per_year(value))
-        return value
+        if value is not None:
+            if isinstance(value, str):
+                value = parse_percentage_per_year(value)
+            else:
+                value = PercentagePerYear(value)
+            value = str(value)
+        return super().get_prep_value(value)
 
     def get_internal_type(self):
         return "CharField"