diff --git a/addons/mrp/mrp.py b/addons/mrp/mrp.py
index 8a25af01e7b..ed3872a855a 100644
--- a/addons/mrp/mrp.py
+++ b/addons/mrp/mrp.py
@@ -786,6 +786,7 @@ class mrp_production(osv.osv):
         for wc_line in production.workcenter_lines:
             wc = wc_line.workcenter_id
             if wc.costs_journal_id and wc.costs_general_account_id:
+                # Cost per hour
                 value = wc_line.hour * wc.costs_hour
                 account = wc.costs_hour_account_id.id
                 if value and account:
@@ -799,9 +800,9 @@ class mrp_production(osv.osv):
                         'ref': wc.code,
                         'product_id': wc.product_id.id,
                         'unit_amount': wc_line.hour,
-                        'product_uom_id': wc.product_id.uom_id.id
+                        'product_uom_id': wc.product_id and wc.product_id.uom_id.id or False
                     } )
-            if wc.costs_journal_id and wc.costs_general_account_id:
+                # Cost per cycle
                 value = wc_line.cycle * wc.costs_cycle
                 account = wc.costs_cycle_account_id.id
                 if value and account:
@@ -815,7 +816,7 @@ class mrp_production(osv.osv):
                         'ref': wc.code,
                         'product_id': wc.product_id.id,
                         'unit_amount': wc_line.cycle,
-                        'product_uom_id': wc.product_id.uom_id.id
+                        'product_uom_id': wc.product_id and wc.product_id.uom_id.id or False
                     } )
         return amount