diff --git a/addons/hr_payroll/hr_payroll.py b/addons/hr_payroll/hr_payroll.py
index 7921fada640..9d14545a462 100644
--- a/addons/hr_payroll/hr_payroll.py
+++ b/addons/hr_payroll/hr_payroll.py
@@ -474,7 +474,9 @@ class hr_payslip(osv.osv):
         def _sum_salary_rule_category(localdict, category, amount):
             if category.parent_id:
                 localdict = _sum_salary_rule_category(localdict, category.parent_id, amount)
-            localdict['categories'].dict[category.code] = category.code in localdict['categories'].dict and localdict['categories'].dict[category.code] + amount or amount
+            if category.code in localdict['categories'].dict:
+                amount += localdict['categories'].dict[category.code]
+            localdict['categories'].dict[category.code] = amount
             return localdict
 
         class BrowsableObject(object):