diff --git a/mixin.py b/mixin.py
index abd7a20..04e94f4 100644
--- a/mixin.py
+++ b/mixin.py
@@ -144,12 +144,20 @@ class EdocumentMixin(object):
     def tax_unece_code(self, tax):
         """ 'tax': invoice.line
         """
-        if not (tax.unece_code or ''):
+        unece_code = self.get_tax_unece_code(tax)
+        if not unece_code:
             raise UserError(gettext(
                 'edocument_xrechnung.msg_tax_code_missing',
                 taxname=tax.rec_name))
         return tax.unece_code
 
+    def get_tax_unece_code(self, tax):
+        while tax:
+            if tax.unece_code:
+                return tax.unece_code
+                break
+            tax = tax.parent
+
     def get_category_code(self, tax):
         while tax:
             if tax.unece_category_code: