diff --git a/purchase.py b/purchase.py
index 3c93eb065d..840d84279e 100644
--- a/purchase.py
+++ b/purchase.py
@@ -54,7 +54,7 @@ class RequestQuotation(IncotermMixin, metaclass=PoolMeta):
 
     @classmethod
     def _incoterm_related_party(cls):
-        return Eval('supplier'), ['supplier']
+        return Eval('supplier'), {'supplier'}
 
 
 class RequestCreatePurchase(metaclass=PoolMeta):
diff --git a/sale.py b/sale.py
index ca44a7ff2d..273c85d259 100644
--- a/sale.py
+++ b/sale.py
@@ -102,7 +102,7 @@ class Opportunity(IncotermMixin, metaclass=PoolMeta):
 
     @classmethod
     def _incoterm_readonly_state(cls):
-        return ~Eval('state').in_(['lead', 'opportunity']), ['state']
+        return ~Eval('state').in_(['lead', 'opportunity'])
 
     def _get_sale_opportunity(self):
         sale = super()._get_sale_opportunity()
diff --git a/stock.py b/stock.py
index 0423313395..07f8681724 100644
--- a/stock.py
+++ b/stock.py
@@ -29,7 +29,7 @@ class ShipmentIn(ShipmentMixin, metaclass=PoolMeta):
 
     @classmethod
     def _incoterm_related_party(cls):
-        return Eval('supplier'), ['supplier']
+        return Eval('supplier'), {'supplier'}
 
 
 class ShipmentInReturn(ShipmentMixin, metaclass=PoolMeta):
@@ -37,7 +37,7 @@ class ShipmentInReturn(ShipmentMixin, metaclass=PoolMeta):
 
     @classmethod
     def _incoterm_related_party(cls):
-        return Eval('supplier'), ['supplier']
+        return Eval('supplier'), {'supplier'}
 
 
 class ShipmentOut(ShipmentMixin, metaclass=PoolMeta):
@@ -45,7 +45,7 @@ class ShipmentOut(ShipmentMixin, metaclass=PoolMeta):
 
     @classmethod
     def _incoterm_related_party(cls):
-        return Eval('customer'), ['customer']
+        return Eval('customer'), {'customer'}
 
 
 class ShipmentOutReturn(ShipmentMixin, metaclass=PoolMeta):
@@ -53,4 +53,4 @@ class ShipmentOutReturn(ShipmentMixin, metaclass=PoolMeta):
 
     @classmethod
     def _incoterm_related_party(cls):
-        return Eval('customer'), ['customer']
+        return Eval('customer'), {'customer'}