diff --git a/addons/account/account_analytic_line.py b/addons/account/account_analytic_line.py
index 2ad40592011..43bd821695d 100644
--- a/addons/account/account_analytic_line.py
+++ b/addons/account/account_analytic_line.py
@@ -80,11 +80,10 @@ class account_analytic_line(osv.osv):
         j_id = analytic_journal_obj.browse(cr, uid, journal_id, context=context)
         prod = product_obj.browse(cr, uid, prod_id, context=context)
         result = 0.0
-        product_uom_id=False
-        if prod_id:
-            product_uom_id = prod.uom_id.id
+        if prod_id or not unit:
+            unit = prod.uom_id.id
             if j_id.type == 'purchase':
-                product_uom_id = prod.uom_po_id.id
+                unit = prod.uom_po_id.id
         if j_id.type <> 'sale':
             a = prod.product_tmpl_id.property_account_expense.id
             if not a:
@@ -131,7 +130,7 @@ class account_analytic_line(osv.osv):
         return {'value': {
             'amount': result,
             'general_account_id': a,
-            'product_uom_id':product_uom_id
+            'product_uom_id':unit
             }
         }