diff --git a/maintenance_usage/models/maintenance.py b/maintenance_usage/models/maintenance.py
index 61ce9130..87018698 100644
--- a/maintenance_usage/models/maintenance.py
+++ b/maintenance_usage/models/maintenance.py
@@ -57,7 +57,9 @@ class MaintenanceEquipment(models.Model):
         result = super(MaintenanceEquipment, self).write(values)
         return result
 
-    def _log_usage(self, values={}):
+    def _log_usage(self, values=None):
+        if not values:
+            values = {}
         values['equipment_id'] = self.id
         values['date'] = fields.Datetime.now()
         self.env['maintenance.usage.log'].create(values)