diff --git a/invoice.py b/invoice.py
index 05b5e6c..b2ae950 100644
--- a/invoice.py
+++ b/invoice.py
@@ -20,6 +20,7 @@ class InvoiceLine(metaclass=PoolMeta):
                 Eval('quantity', None) != None,
                 ),
             )
-        cls.unit.depends.extend(['type', 'quantity'])
+        cls.unit.depends.add('type')
+        cls.unit.depends.add('quantity')
 
 # end Invoice
diff --git a/tests/test_invoice.py b/tests/test_invoice.py
index 811b717..55b6051 100644
--- a/tests/test_invoice.py
+++ b/tests/test_invoice.py
@@ -17,3 +17,5 @@ class InvoiceTestCase(ModuleTestCase):
         pass
 
 # end InvoiceTestCase
+
+del ModuleTestCase