diff --git a/purchase.xml b/purchase.xml
index f1976a7915..41b65b99b3 100644
--- a/purchase.xml
+++ b/purchase.xml
@@ -337,7 +337,7 @@
             <field name="act_from" ref="purchase_activity_waiting_invoice_purchase"/>
             <field name="act_to" ref="purchase_activity_invoice_purchase_exception"/>
             <field name="trigger_model">account.invoice</field>
-            <field name="trigger_expr_id">invoices</field>
+            <field name="trigger_expr_id">[x.id for x in invoices]</field>
             <field name="condition">invoice_exception</field>
         </record>
         <record model="workflow.transition" id="purchase_transition_invoice_purchase_exception_invoice_purchase">
@@ -354,7 +354,7 @@
             <field name="act_from" ref="purchase_activity_waiting_invoice_purchase"/>
             <field name="act_to" ref="purchase_activity_invoice_done"/>
             <field name="trigger_model">account.invoice</field>
-            <field name="trigger_expr_id">invoices</field>
+            <field name="trigger_expr_id">[x.id for x in invoices]</field>
             <field name="condition">invoice_paid</field>
         </record>
         <record model="workflow.transition" id="purchase_transition_invoice_done_invoice_method_done">
@@ -429,7 +429,7 @@
             <field name="act_from" ref="purchase_activity_waiting_invoice_packing"/>
             <field name="act_to" ref="purchase_activity_invoice_packing_exception"/>
             <field name="trigger_model">account.invoice</field>
-            <field name="trigger_expr_id">invoices</field>
+            <field name="trigger_expr_id">[x.id for x in invoices]</field>
             <field name="condition">invoice_exception</field>
         </record>
         <record model="workflow.transition" id="purchase_transition_invoice_packing_exception_cancel">
@@ -446,7 +446,7 @@
             <field name="act_from" ref="purchase_activity_waiting_invoice_packing"/>
             <field name="act_to" ref="purchase_activity_invoice_packing_done"/>
             <field name="trigger_model">account.invoice</field>
-            <field name="trigger_expr_id">invoices</field>
+            <field name="trigger_expr_id">[x.id for x in invoices]</field>
             <field name="condition">invoice_paid</field>
         </record>
         <record model="workflow.transition" id="purchase_transition_invoice_packing_done_invoice_packing_method_done">