diff -r 6c85f394741c invoice.py
--- a/trytond/trytond/modules/account_invoice/invoice.py	Wed Jan 24 20:14:28 2018 +0100
+++ b/trytond/trytond/modules/account_invoice/invoice.py	Wed Feb 07 15:39:49 2018 +0100
@@ -1020,6 +1020,11 @@
             invoice.number = invoice.get_next_number()
         cls.save(invoices)

+    def invoice_type_criteria(self):
+        if self.untaxed_amount < 0:
+            return '_credit_note'
+        return '_invoice'
+
     def get_next_number(self, pattern=None):
         pool = Pool()
         Sequence = pool.get('ir.sequence.strict')
@@ -1040,11 +1045,7 @@
         pattern.setdefault('fiscalyear', fiscalyear.id)
         pattern.setdefault('period', period.id)
         invoice_type = self.type
-        if (all(l.amount < 0 for l in self.lines if l.product)
-                and self.total_amount < 0):
-            invoice_type += '_credit_note'
-        else:
-            invoice_type += '_invoice'
+        invoice_type += self.invoice_type_criteria()

         for invoice_sequence in fiscalyear.invoice_sequences:
             if invoice_sequence.match(pattern):