diff --git a/invoice.py b/invoice.py
index ead258f12a..3d5d993b36 100644
--- a/invoice.py
+++ b/invoice.py
@@ -2209,7 +2209,7 @@ class InvoiceReport(Report):
         if invoice.state in ('open', 'paid'):
             invoice_obj.write(invoice.id, {
                 'invoice_report_format': res[0],
-                'invoice_report_cache': res[1],
+                'invoice_report_cache': buffer(res[1]),
                 })
         return res