diff -r 69b5dc36b783 trytond/trytond/modules/account_invoice/invoice.py
--- a/trytond/trytond/modules/account_invoice/invoice.py	Mon Feb 13 12:47:07 2017 +0100
+++ b/trytond/trytond/modules/account_invoice/invoice.py	Mon Feb 13 12:48:02 2017 +0100
@@ -965,6 +965,11 @@
         move.lines = move_lines
         return move

+    def invoice_type_criteria(self):
+        if self.untaxed_amount < 0:
+            return '_credit_note'
+        return '_invoice'
+
     def set_number(self):
         '''
         Set number to the invoice
@@ -986,10 +991,7 @@
             date=accounting_date, test_state=test_state)
         period = Period(period_id)
         invoice_type = self.type
-        if all(l.amount <= 0 for l in self.lines):
-            invoice_type += '_credit_note'
-        else:
-            invoice_type += '_invoice'
+        invoice_type += self.invoice_type_criteria()
         sequence = period.get_invoice_sequence(invoice_type)
         if not sequence:
             self.raise_user_error('no_invoice_sequence', {