diff --git a/addons/hr_expense/hr_expense.py b/addons/hr_expense/hr_expense.py
index fca62261814..44ecef2ebba 100644
--- a/addons/hr_expense/hr_expense.py
+++ b/addons/hr_expense/hr_expense.py
@@ -290,15 +290,14 @@ class hr_expense_line(osv.osv):
 
     def onchange_uom(self, cr, uid, ids, product_id, uom_id, context=None):
         res = {'value':{}}
-        if not uom_id:
+        if not uom_id or not product_id:
             return res
-        if product_id:
-            product = self.pool.get('product.product').browse(cr, uid, product_id, context=context)
-            uom = self.pool.get('product.uom').browse(cr, uid, uom_id, context=context)
-            if uom.category_id.id != product.uom_id.category_id.id:
-                res['warning'] = {'title': _('Warning'), 'message': _('Selected Unit of Measure does not belong to the same category as the product Unit of Measure')}
-                uom_id = product.uom_id.id
-        res['value'].update({'uom_id': uom_id})
+        product = self.pool.get('product.product').browse(cr, uid, product_id, context=context)
+        uom = self.pool.get('product.uom').browse(cr, uid, uom_id, context=context)
+        if uom.category_id.id != product.uom_id.category_id.id:
+            res['warning'] = {'title': _('Warning'), 'message': _('Selected Unit of Measure does not belong to the same category as the product Unit of Measure')}
+            uom_id = product.uom_id.id
+            res['value'].update({'uom_id': uom_id})
         return res
 
 hr_expense_line()