diff --git a/invoice.py b/invoice.py
new file mode 100644
index 0000000..2a1928e
--- /dev/null
+++ b/invoice.py
@@ -0,0 +1,16 @@
+from trytond.pool import Pool, PoolMeta
+from trytond.model import ModelView, fields
+from trytond.exceptions import UserError
+from trytond.pyson  import Eval
+
+
+class InvoiceLine(metaclass=PoolMeta):
+    __name__ = 'account.invoice.line'
+
+    @classmethod
+    def __setup__(cls):
+        super(InvoiceLine, cls).__setup__()
+
+        cls.product.states['required'] = (Eval('type') == 'line')
+        cls.unit_price.domain = []
+