diff --git a/invoice.py b/invoice.py
index e482fd0..67edc85 100644
--- a/invoice.py
+++ b/invoice.py
@@ -88,7 +88,7 @@ class Invoice(metaclass=PoolMeta):
     def _set_sii_keys(self):
         tax = None
         for t in self.taxes:
-            if t.tax.sii_book_key:
+            if t.tax and t.tax.sii_book_key:
                 tax = t.tax
                 break
         if not tax: